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NCERT Exemplar for Class 12 Maths - Application Of Derivatives - Free PDF Download

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Free PDF download of NCERT Exemplar for Class 12 Maths Chapter 6 - Application Of Derivatives solved by Expert Maths teachers on Vedantu.com as per NCERT (CBSE) Book guidelines. All Chapter 6 - Application Of Derivatives Exercise questions with solutions to help you to revise the complete syllabus and score more marks in your Examinations.

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Access NCERT Exemplar Solutions for Grade 12 Mathematics Chapter 6.- Application of Derivatives

Solved Examples

Short Answer Type (S.A.)

1. For the curve y =5x-2x3 if x increases at the rate of 2 units/sec, then how fast is the slope of curve changing when x = 3?

Ans: Here x is increasing at the rate of 2unit/sec, so we can write this as dxdt=2

Now we find the slope of the curve which is dydx=5-6x2

We have asked to find the change in slope which can we written mathematically as:

 ddt(dydx)  -12xdxdt                [dxdt=2](given)

ddt(dydx)  -12×3×2

ddt(dydx)  -72unit/sec

-sign represents that slope is decreasing. Hence the slope will decrease at the rate of 72unit/sec.


2. Water is dripping out from a conical funnel of semi-vertical angle 4πat the uniform rate of 2 cm2 /sec in the surface area, through a tiny hole at the vertex of the bottom. When the slant height of the cone is 4 cm, find the rate of decrease of the slant height of water. 

Ans: 


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Given- dsdt=2cm2/secWhere s= surface area

and dsdt= decrease in surface area

We know that the surface area of cone is s= π rl

Now we reform this relation in terms of  s and lonly.

So, sin π 4=rl

r=l2

Now, s= π l22

dsdt=2 π ldldt

When l=4, dsdt=42 π dldt.

dldt=242 π     [On substituting the value of dsdt=2]

dldt=24 π cm/sec


3. Find the angle of intersection of the curves y2=xand y=x2.

Ans: First we find the intersection points of the given curve and then find the angle between the slope of curves on the intersection points. So first we solve both the equations:

y2=x-------(1)

y=x2-------(2)

From eq. 1 and 2 

x4=xx4x=0

x(x31)=0

x=0,   x=1Now putting these values in equation 2 we get the value of y as

y=0,y=1   Hence the intersection points (0,0) and (1,1)

Now slope of tangent on curve y2=x is 2ydydx=1dydx=12y

and slope of tangent on curve x2=  is dydx=2x

At (0,0) slope of y2=xis dydx= which means the tangent on this curve is parallel to y-axis and slope of tangent on curve y=x2is dydx=0which is parallel to x-axis. That means the tangents are perpendicular on each other.

Hence angle of intersection at point (0,0) = π 2

At point (1,1) slope of tangent on curve y2=x dydx=12and that of x2=ydydx=2

Angle b/w two lines tan θ =|m1m21+m1m2|

tan θ =|2121+2.12|

tan θ =34

 θ =tan1(34)


4. Prove that the function f(x) = tanx – 4x is strictly decreasing on (π3,π3).

Ans: We know that a function is strictly decreasing if f′(x)<0 in the given interval.

Given: π 3x π 3

Here f′(x)=sec2x4

For given interval

 1<secx<2

1<sec2x<4

3<sec2x4<0

Thus for  π 3<x< π 3f′(x)<0 that means function is strictly decreasing in the given interval.


5. Determine for which values of x, the function y=x44x33is increasing and for which values, it is decreasing. 

Ans: Here we first find the derivative of the given function as:

dydx=4x34x2

Now we put this value equal to 0 for finding the critical points such as:

4x34x2=0

4x2(x1)=0

x=0,x=1


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Since f’(x) is continuous in the interval (,0]and [,1]and f’(x)<0 in (,0)(0,1) Hence the function is decreasing in (,1] and will be increasing [1,).


6. Show that the function f(x)=4x318x2+27x7 has neither maxima nor minima. 

Ans: f(x)=4x318x2+27x7

 (x)=12x236x+27

 (x)=3(4x212x+9)

For critical point  (x)=0

(4x212x+9)=0

(2x3)2=0

x=32 This is the critical point.

  (x)=3(8x12)   at x=32  (32)=3(1212)=0

  (32)=3(8)

Here at  x=32The 2nd and 3rd derivative is constant, which means the function's value is not changing . Hence we can say the function neither has maxima nor minima.


7. Using differentials, find the approximate value of 0.082

Ans: Let f(x)=x

We know that,

 (x)=f(x+ Δ x)f(x) Δ x

Here we write 0.082 as = 0.09-0.008 =(x+ Δ x), on comparing we get x=0.09 and  Δ x=0.008

f(x+ Δ x)=f(x)+ Δ x.   f  (x)

f(0.090.008)=f(0.09)+(0.008).   f  (0.09)

0.082=0.09+(0.008).   120.09                   [f(x)=   (x)=12x]

0.082=0.30.0080.6

=0.30.0133=0.2867


8. Find the condition for the curves x2a2y2b2=1;xy=c2 to intersect orthogonally. 

Ans: For the curves becoming orthogonal the slope of tangents on the curves should be perpendicular like m1×m2=1

Let the curves intersect at point(h,k). So now we find the slope of tangents on both the curves,

x2a2y2b2=1

2xa22yb2dydx=0                      [On differentiating both sides]

dydx=b2a2xy

m1=b2a2hk 

Now using the curve xy=c2

y+xdydx=0

dydx=yx

m2=kh

For orthogonality the condition is  m1×m2=1

m1×m2=1

b2a2hk×kh=1

b2a2=1

a2b2=0


9. Find all the points of local maxima and local minima of the function f(x)=34x48x3452x2+105.

Ans: f(x)=34x48x3452x2+105

 (x)=3x324x245x

 (x)=3x(x2+8x+15)

 (x)=3x(x+5)(x+3)

Now we find the critical points as  (x)=0

3x(x+5)(x+3)=0

x=0,x=5,x=3

For Finding maxima and minima we find the second derivative as:

f(x)=9x248x45

On substituting x=0,-5,-3 in f’’(x) the function will have local maxima if f’’(x)

<0 and will have local minima if f’’(x)>0.

f(0)=45<0. Therefore at x=0 function has local maxima

f(5)=9(5)248(5)45=30<0. Therefore at x=-5 function has local maxima.

f(3)=9(3)248(3)45=18>0. Therefore at x=-3 function has local minima.


10. Show that the local maximum value of x+1x is less than the local minimum value.

Ans: Given f(x)=x+1x

For finding the local maxima and minima we first find out the first derivative, after solving the f’(x) we get critical points and for those critical points we check the second derivative of the function whether it is negative or positive.

f(x)=x+1x

 (x)=11x2

Now,  (x)=0

11x2=0

x=±1

  (x)=ddx(f(x))

   (x)=2x3

   (1)=21=2>0Therefore the function has local minima at x=1and the local minimum value is   (1)=2

  (1)=21=2<0 Therefore the function has local maxima at x=-1 and the local maximum value is f(1)=2

Hence Local maximum value(-2) is less than local minimum value(2).


Long Answer Type (L.A.)

11. Water is dripping out at a steady rate of 1 cu cm/sec through a tiny hole at the vertex of the conical vessel, whose axis is vertical. When the slant height of water in the vessel is 4 cm, find the rate of decrease of slant height, where the vertical angle of the conical vessel is  π 6.

Ans: 


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Here we are given  that the water is dripping out that means = dvdt=1cm3/sec.   and we have to find out dldt=?

We know that volume of a cone V=13 π r2h-------------(1)

From the figure:

cos π 6=hl

h=  cos30=3l2---(2)

sin( π 6)=rl

r=  sin30=l2------(3)

Putting the values of r & h in eq. 1 

V=13 π r2h

V=13 π (l2)2(3l2)

dVdt=13 π 383l2.dldt

1=3 π 8(4)2.   dldt

   dldt=123 π cm/sec

Therefore, the rate of decrease of slant height is    dldt=123 π cm/sec.


12. Find the equation of all the tangents to the curve y=cos(x+y), –2π ≤ x ≤ 2π, that is parallel to the line x+2y=0.

Ans: 

Given: y=cos(x+y)

dydx=sin(x+y){1+dydx}

dydx[1+sin(x+y)]=sin(x+y)

dydx=sin(x+y)[1+sin(x+y)]

This is the slope of the tangent to the given curve.

Now we have a line x+2y=0whose slope is m=12and we know that if two lines are parallel then their slopes are equal. In the problem we are given that the tangent line is parallel to the x+2y=0.

Since we equate the slopes of line and tangent as:

sin(x+y)[1+sin(x+y)]=12

2sin(x+y)=1sin(x+y)

sin(x+y)=1(1) and y=cos(x+y)   (Given)-------------(2)

After squaring  on both side in eq. 1 and eq.2 and adding them

sin2(x+y)+cos2(x+y)=1+y2   

1=1+y2

y=0

Now putting this value in eq.2 

cosx=0

x=(2n+1) π 2where   n=0,±1,±2.....

Given that 2 π x2 π . So we select only those values of x which belong into the given range. Thus x=± π 2,±3 π 2but x= π 2,3 π 2 does not satisfy eq. 1

Hence the points are ( π 2,0),(3 π 2,0).

Now the equation of tangent passing through the point ( π 2,0)and having slope 12is (y0)=12(x π 2)2x+4y π =0 and equation of tangent passing through the point (3 π 2,0)and having slope 12 is 

(y0)=12(x+3 π 2)2x+4y+3 π =0.


13. Find the angle of intersection of the curves y2=4ax and x2=4by.

Ans: Given y2=4ax& x2=4by

The angle of intersection of the curve means the angle between the tangents on both curves. First we find the intersection point for both the curves.

(x24b)2=4ax

x4=64ab2x

x464ab2x=0

x(x364ab2)=0

x=0,   x=4a(13)b(23)

Substituting these values in y2=4axwe get y=0,   y=4a23b13. Thus the intersection points are (0,0)   &(4a(13)b(23),4a23b13)

y2=4ax

2ydydx=4a

dydx=2ay

And x2=4by

2x=4bdydx

dydx=x2b

At (0,0) the slope of tangent to the curve y2=4axis which means the tangent is parallel to y-axis and tangent to the curve x2=4byis 0 which means parallel to the x-axis. Thus the angle b/w these curves =  π 2.

At (4a(13)b(23),4a23b13)the slope of tangent to the curve y2=4axis m1=12(ab)13

And the slope of tangent to the curve x2=4byis m2=2(ab)13

Therefore the angle b/w the curve is tan θ =|2(ab)1312(ab)131+12(ab)13×2(ab)13|=3(a)13(b)132(a23+b23)

 θ =tan1(3(a)13(b)132(a23+b23))


14. Show that the equation of normal at any point on the curve x=3cosθcos3θ, y=3sinθsin3θis 4(ycos3θxsin3θ)=3sin4θ.

Ans: x=3cos θ cos3 θ 

dxθ =3sin θ +3cos2 θ .sin θ 

dxθ =3sin θ (1cos2 θ )

dxθ =3sin3 θ ----------------(1)

Now, y=3sin θ sin3 θ 

dyθ =3cos θ 3sin2 θ .cos θ .dyθ =3cos θ (1sin2 θ )

dyθ =3cos3 θ -----------------(2)

Now we find the slope as dydx=dyθ dxθ =3cos3 θ 3sin3 θ . Since the slope of the normal =1dydx=sin3 θ cos3 θ 

The equation of line passing through a point (3cos θ cos3 θ ,   3sin θ sin3 θ )and having slope sin3 θ cos3 θ is [y(3sin θ sin3 θ )]=sin3 θ cos3 θ [x(3cos θ cos3 θ )]

y.cos3 θ 3sin θ .cos3 θ +sin3 θ .cos3 θ =xsin3 θ 3cos θ .sin3 θ +sin3 θ .cos3 θ 

y.cos3 θ xsin3 θ =3sin θ cos θ (cos2 θ sin2 θ )

We know that cos2 θ =cos2 θ sin2 θ 

y.cos3 θ xsin3 θ =32×2sin θ cos θ (cos2 θ ) [Multiply and divide by 2]

y.cos3 θ xsin3 θ =32×sin2 θ (cos2 θ )

y.cos3 θ xsin3 θ =34×sin4 θ         [Multiply and divide by 2]

4(y.cos3 θ xsin3 θ )=3sin4 θ 

Hence the equation of normal on the curve is 4(y.cos3 θ xsin3 θ )=3sin4 θ .


15. Find the maximum and minimum values of f(x)=secx+log cos2x, 0 < x < 2π.

Ans: Here for finding the maxima and minima, first we find the critical points.

f(x)=secx+log   cos2x

f  (x)=secx.   tan   x+1cos2x2cosx.(sinx)                        [ddxsecx=secx.tanx]

f  (x)=tanx(secx2)

Now for critical points,  (x)=0

tanx=0 and (secx2)=0secx2=0

x=0, π   and     x= π 3,5 π 3

f  (x)=tanx.   secx2tanx

f   (x)=secx.sec2x+secx.   tan2x2sec2x     [On usingddx(u.v)=v.dudx+u.dudx]

f   (x)=sec3x+secx.   tan2x2sec2x   

Here on  putting x=0, π ,    π 3,5 π 3

Here we get the maxima when f’’(x)<0 and minima when f’’(x)<0.

  (0)=sec3x+secx.   tan2x2sec2x=1+02=1<0Therefore at x=0 function has the maxima.

At x= π ,   ( π )=sec3x+secx.   tan2x2sec2x=1+02=3<0.Therefore function has the maxima at x= π .

At x= π 3,   ( π 3)=sec3x+secx.   tan2x2sec2x=(2)3+2.(3)2.(4)=6>0. Therefore function has minima at x= π 3.

At x=5 π 3,   (5 π 3)=sec3x+secx.   tan2x2sec2x=(2)3+2.(3)2.(4)=6>0. 

Therefore function has minima at x=5 π 3

SO from the above the maxima points are 0,  π and minima points are  π 3, 5 π 3

Now the maximum values of f(x) is f(0)=secx+log   cos2x=1+0=1

f( π )=sec( π )+log   cos2( π )=1+0=1

Minimum value at  π 3, 5 π 3 are f( π 3)=sec( π 3)+log   cos2( π 3)=2+2log(12) and f(5 π 3)=sec(5 π 3)+log   cos2(5 π 3)=2+2log(12).


16. Find the area of the greatest rectangle that can be inscribed in an ellipse x2a2+y2b2=1.


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Ans: From the figure,

Let the side on the major axis is =2x, side on the minor axis = 2y. Now the area of rectangle A=2x.2y=4xySquaring on both sides A2=16x2y2=S

S=16x2y2

S=16x2b2{1x2a2}

Now we have to maximise this function for getting the maximum area of a rectangle.

dSdx=16b2{x2x4a2}

dSdx=16b2{2x4x3a2}

dSdx=16b2a2{2a2x4x3}

For critical points dSdx=0

2a2x4x3=0

x(2a24x2}=0

x=0   (Rejected)   &   x=a2

On putting the value x=a2 in equation of ellipse we get y=b2

Now,  d2Sdx2=16b2a2{2a212x2}

At x=a2, d2Sdx2=16b2a2{4a2}<0.   Therefore we get the maximum area. 

Hence At x=a2,   y=b2. The area of rectangle Amax=2x.2y=4xy=4.a2.b2=2ab.

Maximum area =2ab.Sq. units


17. Find the difference between the greatest and least values of the function f(x)=sin2xx, on [π2,π2] .

Ans: f(x)=sin2xx

 (x)=2cos2x1

For critical points  (x)=0

2cos2x1=0

cos2x=122x= π 3,    π 3x= π 6,    π 6

Now in interval  π 2x π 2we find the values at points  π 6,  π 6,  π 2& π 2

f( π 6)=sin2( π 6)( π 6)=32+ π 6

f( π 6)=sin2( π 6)( π 6)=32 π 6

f( π 2)=sin2( π 2)( π 2)=f( π 6)= π 2

f( π 2)=sin2( π 2)( π 2)= π 2

If we compare all four value the greatest value is  π 2 and least value is  π 2. Hence the difference between them is =  π 2( π 2)= π .


.18. An isosceles triangle of vertical angle 2θ is inscribed in a circle of radius a. Show that the area of triangle is maximum when  θ =π6.


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Ans: Given ABC is an isolated triangle.

We know that area of triangle = 12×Height×Base.

Here Height = AD, Base = BC

BC=2BD=2.asin2 θ 

AD=OA+OD where OA =Radius =a and OD=OBcos2 θ 

Hence AD=a+acos2 θ 

Now Area of triangle  Δ =12×Height×Base.

=12×(a+acos2 θ )×2asin2 θ .

We have to maximise this area of triangle where the area depends on angle  θ ,So we use the 2nd derivative test for finding the maximum area. 

 Δ =12×a2(1+cos2 θ )×2sin2 θ 

 Δ =12×a22sin2 θ +12×a2×sin4 θ 

Δ θ =a2×2cos2 θ +2a2×cos4 θ 

For critical points Δ θ =0

a2×2cos2 θ +2a2×cos4 θ =0

cos2 θ =cos4 θ 

cos2 θ =cos( π 4 θ )       [cos( π  θ )=cos θ ]

2 θ =( π 4 θ )

 θ = π 6

d2 Δ d θ 2=4a2sin2 θ 8a2sin2 θ 

d2 Δ d θ 2=4a2(sin2 θ +2sin2 θ )<0

At  θ = π 6d2 Δ d θ 2=4a2(sin π 3+2sin π 3)=4a2332<0Therefore the area of triangle is maximum when  θ = π 6.


Objective Type Questions

Choose the correct answer from the given four options in each of the following Examples 19 to 23. 

19. The abscissa of the point on the curve 3y=6x5x3, the normal at which passes through origin is: 

(A) 1 

(B) 13

(C) 2

 (D) 12

Ans: Given: 3y=6x5x3

Let the point on the curve is (x1,   y1)

So the curve is 3y1=x1(65x12)y1x1=(65x12)3-------------(1)

Now we find the Slope of normal at this point. 

3y=6x5x3

3dydx=615x2

dydx=25x2

Slope of Normal = 125x2

Equation of normal Passing through origin (y0)=125x2(x0)

Now the Equation of normal on point (x1,   y1)is y1=x125x12y1x1=125x12-----(2)

By using eq. 1 & 2

(65x12)3=125x12

1230x1210x21+25x14=3

Let x12=t

1230t10t+25t2=3

25t240t+15=0

5t28t+3=0

5t25t3t+3=0

5t(t1)3(t1)=0

t=1and   t=35

   t=35(Rejected)

t=1x12=1x1=±1

x1=1 Only satisfies the equations, Hence the option A is the correct answer.


20.The two curves x33xy2+2=0 and 3x2yy3=2

(A) touch each other 

(B) cut at right angle 

(C) cut at an angle  π 3

(D) cut at an angle  π 4

Ans: First we find the slope of both the curves.

x33xy2+2=0

Differentiating on both sides 

3x23y26xy.dydx=0

.dydx=3x23y26xy=x2y22xy=M1---------(1)

Curve2:   3x2yy3=2

Differentiating on both sides

6xy+3x2dydx3y2dydx=0

dydx=6xy3x23y2

dydx=2xyx2y2=M2---------(2)

By Eq. 1 & 2 We can write M1M2=   1.Hence the curves cut at right angles.


21. The tangent to the curve given by x=et.cost, y=et. sint at t=π4 makes with x-axis an angle: 

(A) 0 

(B)  π 4

(C)  π 3 

(D)  π 2

Ans: Slope of the tangent of the curve is dydx

x=et.cost

dxdt=cost.   etetsint

y=et.   sint

dydt=cost.   et+etsint

dydx=dydtdxdt=et(cost+sint)et(costsint)

Slope at point t= π 4, (dydx)t= π 4=(cost+sint)(costsint)=20= Since the tangent is y-axis. 

Hence the tangent (y-axis) will make angle  π 2 with x-axis as the y-axis and x-axis are perpendicular to each other.


22. The equation of the normal to the curve y = sinx at (0, 0) is: 

(A) x=0 

(B) y=0

(C) x+y=0

(D) xy=0

Ans: Slope of the tangent = dydx

Slope of the Normal = 1slope   of   tangent=1dydx----(1)

dydx=cosx

(dydx)(0,0)=cos(0)=1

Slope of the Normal = 11=1

Equation of normal having slope -1, passing through (0,0) is

(y0)=1(x0)

x+y=0

Hence option c is the correct answer.


23. The point on the curve y2=x, where the tangent makes an angle of  π 4with x-axis is 

(A) (12,14)     

(B) (14,12)   

(C) (4, 2) 

(D) (1, 1)

Ans: Given: y2=x

Slope of the tangent of curve y2=xis dydx

So on differentiating the given curve y2=x on both sides 

2y.dydx=1

dydx=12y

Let the point on curve is (x1,y1)then y12=x1and the slope on the this point is dydx=12y1. We are given that the tangent is making the angle of   π 4with x-axis hence the slope will be = tan( π 4)=1. 

dydx=12y1=1

y1=12

On Putting this value in curve equation we get y12=x1x1=(12)2=14

Hence the point is (14,12).Option B is the correct answer.


24. The values of a for which y=x2+ax+25 touches the axis of x are______. 

Ans: The curve touches the x-axis that means the tangent on the curve will touch the x-axis. So we first find the slope and equate it with 0.

dydx=2x+a 

2x+a=0

x=a2

Therefore, 

Now on putting x=a2 in the curve then y- will be zero such as (a2)2+a(a2)+25=0

a24a22+25=0

a22a2+254=0

a2=100

a=±10

Hence the values of a are ±10.


25. If f(x)=14x2+2x+1, then its maximum value is _______.

Ans: For the function f(x) to be maximum 4x2+2x+1should be minimum. Now our aim is to find the minimum value of  4x2+2x+1.

Let g(x)=4x2+2x+1

 (x)=8x+2

For critical points  (x)=0

8x+2=0

x=14

  (x)=8>0Therefore the function 4x2+2x+1has minima at x=14

Since gmin(x)=4x2+2x+1=4.(14)2+2(14)+1=1412+1=34

Hence fmax(x)=34


26. Let f have a second derivative at c such that f(c)=0 and f(c)>0, then c is a point of ______.

Ans: We know that if we have a function f such that at c,  (c)=0 then c is called critical point and then if    (c)>0then point c is called the local minima.


27. Minimum value of f if f(x)=sinx in [π2,π2] is _____.

Ans: We know that sin function 

Given: f(x)=sinx

 (x)=cosx

For critical points  (x)=0

cosx=0

x= π 2, π 2[ π 2, π 2]

  (x)=sinx

At x= π 2,   (x)=sin( π 2)=1<0.   Therefore function has maxima at x= π 2

At  x= π 2,   (x)=sin( π 2)=1>0. Therefore function has maxima at x= π 2

Hence fmin(x)   =sin( π 2)=   1.


28. The maximum value of sinx + cosx is _____.

Ans: Let f(x)=sinx+cosx

 (x)=cosxsinx

For critical points,  (x)=0

cosxsinx=0

cosx=sinx

tanx=1

x= π 4

  (x)=sinxcosx

At x= π 4,   ( π 4)=1212=2     <0. Therefore function has maxima at x= π 4. Hence the maximum value of the function fmax(x)=sin( π 4)+cos( π 4)=12+12=2


29. The rate of change of volume of a sphere with respect to its surface area, when the radius is 2 cm, is______. 

Ans: Volume of a sphere is V= 43 π r3 and surface area is S=4 π r2.

Here the rate of change of volume w.r.t surface will be = dVdS We know that both the quantities(area, volume) depends on radius so we write this rate of change in terms of r as dVdS=dVdrdSdr---------(1)

V=43 π r3dVdr=43.3 π r2=4 π r2-----(2)

S=4 π r2dSdr=8 π r--------(2)

Using the values from Eq.(2)&(3) in Eq.(1)

dVdS=dVdrdSdr=4 π r28 π r=r2

When r=2

dVdS=22=1


EXERCISE 

Short Answer (S.A.)

1. A spherical ball of salt is dissolving in water in such a manner that the rate of decrease of the volume at any instant is proportional to the surface. Prove that the radius is decreasing at a constant rate.

Ans: Given that the rate of decrease of the volume at any instant is proportional to the surface . Let the radius of the spherical ball is r.

Volume of sphere V= 43 π r3dVdt=43.3 π r2[-sign represent that the volume is decreasing]

Surface area of sphere S=4 π r2

According to the problem dVdt α   S

ddt(43 π r3) α   4 π r2

43.3 π r2.   drdt   α   4 π r2

   drdt α   4 π r24 π r2

   drdt=   k.1   [k= proportionality constant]

Since drdt=k. Hence we can say that the radius is decreasing at a constant rate.


2. If the area of a circle increases at a uniform rate, then prove that perimeter varies inversely as the radius.

Ans: Given that the area of the circle is increasing at a uniform rate as dAdt=k-------(1) .

Let the radius of a circle =r

Area of a circle A= π r2

dAdt=2 π r.   drdt-----(2)

By Eq. 1 & 2

k=2 π r.drdt

drdt=k2 π r----(3)

Let the perimeter P=2 π r

dPdt=2 π drdt

Putting the values of drdtby Eq. 3 in above eq.

dPdt=2 π .k2 π r

dPdt=kr

dPdt α   1r

Hence the perimeter varies inversely as the radius.


3. A kite is moving horizontally at a height of 151.5 meters. If the speed of the kite is 10 m/s, how fast is the string being let out; when the kite is 250 m away from the boy who is flying the kite? The height of the boy is 1.5 m. 

Ans: 


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Height(h)=151.5m, Speed of kite =10m/s

From the figure, Let CD be the height of the kite and AB is the height of the boy.

Let BD=  m   =EA   and   AC=250m

dxdt=10m/sec

EC=CDDE=151.51.5=150m

Using pythagoras theorem in  Δ AEC

AC2=AE2+EC2

Here AC=250m , AE= x , EC=150m

y2=x2+1502

When y=250

2502=x2+1502

25021502=x2

x2=(250150)(250+150)

x2=(250150)(250+150)

x2=100×400

x=20   and   20 [side can’t be negative, so x=-20 rejected]

Now , y2=x2+1502

Differentiating on both sides

2y.dydt=2x.dxdt

dydt=xy.dxdt

On putting the values x=20, y=250 and dxdt=10, we get

dydt=200250.10=8m/sec

So, the string being let out at the rate of 8m/sec.


4. Two men A and B start with velocities v at the same time from the junction of two roads inclined at 45° to each other. If they travel by different roads, find the rate at which they are being seperated.

Ans: 


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Let the men A and B start from point c with the 

Same velocities v at the same time. 

And given, ACB=45

Since, Men A and B moving with same velocities v at the same time , so they will cover the same distance, 

Hence AC=AB, Therefore the triangle ABC is an isosceles triangle. 

Now we draw CDAB

Let at time t the distance b/w A and B is y and AC=x

 Δ ABCis an isosceles triangle, So AC=AB=x

In  Δ ACDand  Δ BCD

CAB=CBD    ( AC=AB)

ADC=BDC=90

CD=CD (common side)

By ASA ,   Δ ACD Δ BCD

ACD=DCB

Since, ACD+DCB=45

 ACD=DCB=12. π 4= π 8

In  Δ ACD

sin( π 8)=ADAC

sin( π 8)=y2x

y=2x.sin( π 8)(1)

Since distance b/w A and B is y. Hence the rate of the separation of A and B is dydt. 

Differentiating the equation 1 w.r.t. t,

dydt=2sin( π 8).   dxdt

dydt=2sin( π 8).   v      [dxdt=v]

dydt=2v.222

dydt=v.22unit/sec

So the rate isv.22at which A and B are being separated.


5. Find an angle θ, 0<θ<π2 , which increases twice as fast as its sine.

Ans: Let the angle  θ   increases twice as fast as its sine as

 θ =2sin θ 

Now,on differentiatingboth sides w.r.t. t, we get

θ dt=2cos θ .θ dt

1=2cos θ .

 θ = π 3

So the required angle is  θ = π 3


6. Find the approximate value of (1.999)5

Ans: Let y+ Δ y=(1.999)5

We write 1.999 as x+ Δ x=1.999then y+ Δ y=(x+ Δ x)2

Let x=2 then  Δ x=1.9992=   0.001

y=(x)5

Differentiating on both sides w.r.t. T, we get 

dydt=5x4dxdt

dy=5x4dx

 Δ y=5x4 Δ x

 Δ y=5(2)4(0.001)

 Δ y=0.080

Now y+ Δ y   =(2)50.080

y+ Δ y   =320.080=31.920


7. Find the approximate volume of metal in a hollow spherical shell whose internal and external radii are 3 cm and 3.0005 cm, respectively. 

Ans: Let the external radius of spherical shell R=3.0005cmand internal radius r=3cm   

Volume of hollow spherical shell V=43 π (R3r3) 

V=43 π (3.0005333)-------(i)

We can find the value of 33easily but the value of 3.00053is tough to find so we use the approximation.

Let y+ Δ y=(3.0005)3 and 3.0005=x+ Δ xwhere x=3,   Δ x=0.0005

Also let y=(x)3

Now we differentiate on both sides w.r.t. t, 

dydt=3x2.dxdt

 Δ y=3x2. Δ x

 Δ y=27×0.0005=0.0135 and y=(3)3=27

So now y+ Δ y=(3.0005)3=27+0.0135=27.0135

Since V=43 π (27.013527)=43 π ×0.0135=0.0180 π   cm3


8. A man, 2m tall, walks at the rate of 123m/s towards a street light which is 513m above the ground. At what rate is the tip of his shadow moving? At what 136 MATHEMATICS rate is the length of the shadow changing when he is 313 m from the base of the light?

Ans: 


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From the figure, Let AB be the height of street light source and CD be the height of man, CD=2m.

Let BC=  m,   CE=  mand the man moving towards the light post so dxdt=123m/sec 

From  Δ ABE   and   Δ DCE,   we see that

ABCD=BECE1632=x+yy

166=x+yy

10y=6x

y=35x

On differentiating both sides w.r.t. T

dydt=35dxdt

dydt=35.(53)=1m/sec

Let the distance of tip of shadow from the foot of street light post is Z

z=x+y

Now on differentiating both sides w.r.t. T, we get 

dzdt=dxdt+dydt

dzdt=   531=83m/sec

Therefore the tip of shadow is moving at the rate of 83m/sectowards the light post and length of the shadow is decreasing at the rate of 1 m/s. 


9. A swimming pool is to be drained for cleaning. If L represents the number of litres of water in the pool t seconds after the pool has been plugged off to drain and L=200(10t)2 . How fast is the water running out at the end of 5 seconds? What is the average rate at which the water flows out during the first 5 seconds?

Ans: Given that L represents the number of liters of water in the pool t seconds after the pool has been plugged off to drain and that is L=200(10t)2

Let the rate at which water is running out = dLdt

 L=200(10t)2

dLdt=400(10t)

Rate at which the water  is running out at the t=5s 

dLdt|t=5=400(10t)=400(105)=2000L/sec

Since the initial Rate = dLdt|t=0=400(10-0)=4000L/sec

Now the average rate = Initial   Rate+Final   Rate2

=4000+20002=3000L/sec


10. The volume of a cube increases at a constant rate. Prove that the increase in its surface area varies inversely as the length of the side. 

Ans: Let the Side of a cube = x

We know the volume of a cube V=x3 and Surface area S=6x2

Differentiating on both sides w.r.t. t,

dVdt=3x2.dxdt

k=3x2.dxdt [Given volume of a cube increases at a constant rate so dVdt=k]

dxdt=k3x2-------(1)

Also the surface area of cube S=6x2

Differentiating on both sides w.r.t. t,

dSdt=12x.dxdt

Now by putting the value of dxdtfrom eq. 1

dSdt=12x.k3x2

dSdt=4kx

dSdt=4(kx)   [K= proportionality constant]

dSdt α   1x

Hence the increase in its surface area varies inversely as the length of the side. 


11. x and y are the sides of two squares such that y=xx2 . Find the rate of change of the area of the second square with respect to the area of the first square.

Ans: Given : x and y are the sides of two squares such that y=xx2

Let the area of first square having side x is A1=x2

The area of second square having side y is A2=y2=(xx2)2 and we have to find the rate of change of the area of the second area w.r.t.  area of the first square which mathematically can be written as =dA2dA1=dA2dtdA1dt

Now, A1=x2

Differentiating on both sides w.r.t. t,

dA1dt=2x.   dxdt--------(1)

And A2=(xx2)2

Differentiating on both sides w.r.t. t,

dA2dt=2(xx2)(12x)dxdt--------(2)

Dividing Eq. 2 by Eq. 1

dA2dA1=dA2dtdA1dt=2(xx2)(12x)dxdt2x.   dxdt

dA2dA1=2x(1x)(12x)2x

dA2dA1=(1x)(12x)

dA2dA1=(12xx+2x2)

dA2dA1=2x23x+1


12. Find the condition that the curves 2x=y2 and 2xy=k intersect orthogonally.

Ans: Here first we find the point of intersection of given curves.

2x=y2-----(1) 

2xy=k----(2)

y=k2x

Now putting this value in eq. 1

2x=(k2x)2

8x3=k2

x=12k23

Now substituting this value in y=k2x

y=k212k23=k13

Thus the point of intersection of curves is (12k23,k13   )

Orthogonality means the tangents on both the curves are intersecting each other at an angle of  π 2 that means m1×m2=   1   where m1&   m2are the slopes of the curves.

2x=y2

Differentiating on both sides w.r.t. x,

2=2y.dydx

dydx=22y

dydx|(12k23,k13)=22(k13)  = m1(assume)

Also,   2xy=k

Differentiating on both sides w.r.t. X,

2[y+x.dydx]=0

dydx=yx

dydx|(12k23,k13)=k1312k23=k13= m2   (assume)

Since curves intersect orthogonally, So

m1×m2=1

1(k13)×k13=1

2k23=1

2k23=1

2k23=1

k23=2

k2=8

Hence for intersecting the given curve orthogonally the required condition is k2=8


13. Prove that the curves xy = 4 and x2+y2=8 touch each other.

Ans: If the curves touch each other then the slope of tangents on both curves on the touching point are the same . Since we have to prove this condition.

So first we find the touching point. 

Let we have a point (x,y) where the curves intersect.

xy = 4 y=4x---(1)

x2+y2=8----(2)

Now on putting the value of y=4xin Eq. 2 

x2+(4x)2=8

x4+16=8x2

Let x2=t

t28t+16=0

(t4)2=0

t=4

x2=4

x=   ±2

Now from Eq. 1 

y=±42

So the touching points (2,2) & (-2,-2)

Now we find the slopes of both curves:

xy = 4

On differentiating both sides w.r.t. x, 

y+x.dydx=0

dydx=yx=m1(say)

Also, x2+y2=8

Differentiating on both sides w.r.t. x,

2x+2y.dydx=0

dydx=yx=m2(Say)

For point (2,2)

m1=22=   1

m2=22=   1

   m1=m2

Also for point (-2,-2)

m1=22=   1

m2=22=   1

   m1=m2

Thus, for both the intersection points,We see that the slopes of both curves are the same. Hence, the curves touch each other.


14.  Find the coordinates of the point on the curve x+y=4 at which tangent is equally inclined to the axes. 

Ans: Given that we have a curve x+y=4----(1)

Differentiating on both sides w.r.t. x, 

12x+12ydydx=0

dydx=yx

The tangent inclined equally on both axes hence 

dydx=±1

yx=±1

Squaring on both sides we get 

yx=1

y=x----(2)

From eq. 1 , y+y=4

  =2

y=4

Putting this value in eq. 2

x=4

Hence the required coordinates of the point are (4,4).


15. Find the angle of intersection of the curves y=4x2 and y=x2.

Ans: Given that both curves are intersecting each other so first we find the intersection point and then find the angle b/w the tangents on both curves drawn on intersecting points.

y=4x2-----(1)

y=x2-------(2)

From Eq. 1 & 2

4x2=x2

2x2=4

x2=2

x=±2

Now from eq. 1 y=x2y=(2)2=2

So the intersection points are (2,2)   and   (2,2).

Now we find the slopes of both curves

y=4x2

dydx=2x=m1(Say)

y=x2

dydx=2x=m2(Say)

For point (2,2)

m1=2x=22

m2=2x=22

So for point (2,2) tan θ =|m1m21+m1m2|=|2222122.22|=|427|

 θ =tan1(427)

Now for point (2,2)

m1=2x=22

m2=2x=22

Here tan θ =|m1m21+m1m2|=|2222122.22|=|427|

 θ =tan1(427)

Therefore, For both intersecting points we get the angle  θ =tan1(427).


16. Prove that the curves y2=4x and x2+y26x+1=0 touch each other at the point (1, 2).

Ans: If the given curves touch each other then the slope of tangents on both curves at the intersecting point is same. 

Let the slope of curve y2=4xis m1and the slope of curve x2+y26x+1is m2. So for these curves m1=m2

 y2=4x

Differentiating on both sides w.r.t. x,

2y.dydx=4

dydx=2y

dydx|(1,2)=2y=22=1

m1=1

And x2+y26x+1=0

Differentiating on both sides w.r.t. x,

2x+2y.dydx6=0

dydx=62x2y

dydx|(1,2)=3xy=22=1

m2=1

Thus we see that the slope of both the curves are equal toeach other i.e., m1=m2=1 at the point(1,2).


17. Find the equation of the normal lines to the curve 3x2y2=8 which are parallel to the line x + 3y = 4. 

Ans: We are given an equation of a curve 3x2y2=8-----(1)

First we find the slope of normal on the given curve 3x2y2=8

Differentiating on both sides w.r.t. X

6x2ydydx=0

dydx=3xy 

This is the slope of tangent on the given curve but we have to find the slope of normal, and we know that slope   of   normal   =1slope   of   tangent   So the slope of normal on the given curve  = 1dydx=y3x

It is given that the normal is parallel to the line x + 3y = 4. We know that the slope of two parallel lines are the same hence the slope of normal will be equal to the slope of the given line . So the slope of line =ab=13

y3x=13

y=x--------(2)

On substituting y=x in eq. 1, we get 

3x2x2=8

2x2=8

x=2,2

For x=2, from eq. 2 y=2

For x=-2, y=-2

Hence the (2,2) and (-2,2) are the points where the normal at the given curve is parallel to line x + 3y = 4.

Now the Required equations of normals passing through points (2,2) & (-2,2) and having slope of 13 are

(y2)=13(x2)         and   (y+2)=13(x+2)

3(y2)=1(x2)       and       3(y+2)=1(x+2)

3y+x=8       and       3y+x=   8

So the required equations of Normal are 3y+x=   ±8     


18.  At what points on the curve x2+y22x4y+1=0, the tangents are parallel to the y-axis?

Ans: We are given an equation of the curve x2+y22x4y+1=0-------(1)

Differentiating on both sides w.r.t. x,

2x+2y.dydx24dydx=0

dydx(2y4)=2(1x)

dydx=2(1x)2(y2)

This is the slope of tangent which is parallel to y-axis and we know that the slope of y-axis is 10.So the above slope of tangent will be equal to 10.

 2(1x)2(y2)=10

(y2)=0

y=2

On substituting y=2 in eq.1, we get 

x2+42x8+1=0

x22x3=0

x23x+x3=0

(x3)(x+1)=0

x=3,1

Hence the required points are (3,2) &(-1,2) where the tangents are parallel to the y-axis.


19. Show that the line xa+yb=1, touches the curve y=b.exa at the point where the curve intersects the axis of y. 

Ans: Given we have given a curve y=b.exaand linexa+yb=1.

We know that if two curves touch each other at a point then the slope of both the curves at the meeting point should be equal i.e. m1=m2where m1&   m2

are slopes of curves.

The curve y=b.exaintersects the y-axis i.e. x=0

y=b.e0=b

So the point of intersection is (0,b).

Now the slope of curve at point (0,b) is:

dydx=b.exa.(1a)

(dydx)(0,b)=b.e0.(1a)=ba=(m1)---------(1)

Also slope of the line  xa+yb=1at point (0,b) is:

Differentiating the equation of line w.r.t. x,

1a+1b.dydx=0

dydx=ba=(m2)-----(2)

From eq. 1 and eq.2 

m1=m2=ba

Therefore, the line xa+yb=1touches the curve y=b.exaat the point (0,b).


20.  Show that f(x)=2x+cot1x+log(1+x2x) is increasing in R.

Ans: Given function  f(x)=2x+cot1x+log(1+x2x)

A function is said to be increasing function if  (x)>0   in its domain .

f(x)=2x+cot1x+log(1+x2x)

Differentiating the function w.r.t. x,

 (x)=2+11+x2+11+x2x(121+x2.2x1)

 (x)=211+x2+11+x2x(x1+x21+x2)

 (x)=211+x211+x2

 (x)=2(1+x2)11+x21+x2

 (x)=1+2x21+x21+x2

For increasing function,  (x)>0

1+2x21+x21+x2>0

1+2x21+x2>0               [1+x2   is   always   positive]

1+2x2>1+x2

(1+2x2)2>1+x2       [Squaring on both sides]

1+4x2+4x4>1+x2

4x4+3x2>0

x2(4x2+3)>0 

This is true for every value of x, Hence the f(x) is increasing in R.


21.  Show that for a ≥ 1, f(x)=3sinxcosx2ax+b is decreasing in R.

Ans: We have function f(x)=3sinxcosx2ax+b 

We know that a function is said to be decreasing if  (x)<0in its domain.

Differentiating the function w.r.t. x,

 (x)=3cosx+sinx2a

=2[32cosx+12sinx]2a     [Multiplying and dividing by 2]

=2[cos( π 6)cosx+sin( π 6)sinx]2a

=2[cos( π 6x)]2a

We know that 1cosx1So 1cos( π 6x)1

22a2cos( π 6x)2a22a

22a (x)22a

Here we have to prove that the function is decreasing , we know that the function is decreasing if  (x)<0. For the given function we have22a (x)22a so if the maximum value of  (x) is <0   then the function will be decreasing in its domain.

So ,  22a0

2a2

a1

Therefore the function will be decreasing for a1.


22. Show that f(x)=tan1(sinx+cosx) is an increasing function in (0,π4).

Ans: The given function  f(x)=tan1(sinx+cosx)

Differentiating the function

 (x)=11+(sinx+cosx)2(cosxsinx)

 (x)=11+(sinx+cosx)2(cosxsinx)

 (x)=11+sin2x+cos2x+2sinx.cosx(cosxsinx)

 (x)=12(1+sinx.cosx)(cosxsinx)

For increasing function  (x)>0

12(1+sinx.cosx)(cosxsinx)>0

(cosxsinx)>0 [2sinx.cosx is +ve in (0, π 4)]

cosx>sinx

tanx<1   

This is true when x(0, π 4)

So the function is increasing in (0, π 4).


23. At what point, the slope of the curve y=x3+3x2+9x27 is maximum? Also find the maximum slope.

Ans: The curve we have: y=x3+3x2+9x27---------(1)

Slope of tangent on the curve dydx=3x2+6x+9

Now, d2ydx2=6x+6

For critical point, d2ydx2=0

6x+6=0

x=1

For slope to be maximum, ddx(d2ydx2)<0

Now , ddx(d2ydx2)=6<0 Hence the slope will be maximum at x=1.

On substituting x=1 in eq. 1 we get,

y=(1)3+3(1)2+9(1)27=1+3+927=   16

So the point is (1, -16) at which the slope of the curve is maximum .

Slope at x=1,  dydx|x=1=3(1)2+6(1)+9=3+6+9=12

Hence the point is (1, -16) and the maximum slope is 12.


24. Prove that f(x)=sinx+3cosx has maximum value at x=π6.

Ans: 

The function we have : f(x)=sin   x+3cos   x

Differentiating the function w.r.t. X,

 (x)=cosx3sinx

For critical point  (x)=0

cosx3sinx=0

tanx=13

x= π 6

For finding either function has maxima or minima, we again differentiate the  (x)

  (x)=   sinx3cosx

At x= π 6,   ( π 6)=   sin( π 6)3cos( π 6)=123.32=2<0

Thus the function has local maxima and the point of maxima is  π 6.


25. If the sum of the lengths of the hypotenuse and a side of a right angled triangle is given, show that the area of the triangle is maximum when the angle between them is  π 3

Ans: 


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Let the triangle is  Δ ABC   in which BC=y, 

AB=x, AC=h

Given : h+x=k--------(1)

By using the pythagorean theorem in triangle  Δ ABC as 

h2=x2+y2 Here x and y are unknown.

By eq. 1 h=k  

(kx)2=x2+y2

k2+x22kx=x2+y2

y=k22kx------(2)

Now area of triangle  Δ ABC=12AB.BC=12xy

A=12x.k22kx

Now we have to maximize the area of the triangle, So we differentiate w.r.t. x, 

dAdx=12[k22kx.+x12k22kx(02k)]

For critical points, dAdx=0

12[k22kx.+x12k22kx(02k)]=0

k22kxkx=0

x=k3

Again differentiating dAdx   w.r.t.   x, we get

d2Adx2=12[12k22kx.(2k)kk22kxx.   12k22kx(2k)k22kx]

=12[k2x(k22kx)322kk22kx]

=k2x2k(k22kx)(k22kx)32

=3k2x2k3(k22kx)32

At x=k3 we have d2Adx2=k3k3(3)32<0 

So area will be maximum at x=k3.

On putting the value x=k3in eq. 2

y=k22k.k3=k3

So the point is (k3,k3) where we get the maximum area of the triangle. At the given point the value of angle is tan θ   =   yx=k3k3=3

tan θ =3 θ = π 3

Therefore the area of triangle is maximum when the angle is  π 3.


26. Find the points of local maxima, local minima and the points of inflection of the function f(x)=x55x4+5x31. Also find the corresponding local maximum and local minimum values.

Ans: Given function f(x)=x55x4+5x31

Differentiating the function w.r.t. x,

 (x)=5x420x3+15x2

 (x)=5x2(x24x+3)

 (x)=5x2(x23xx+3)

 (x)=5x2(x1(x3)

For critical points  (x)=0

5x2(x1(x3)=0

x=0,1,3


(Image will be uploaded soon)


If we see at point x=0 , the value of f’(x) is not changing that means the function has neither maxima nor minima.

At x=1, the value of f’(x) is changing from +ve to -ve so at x=1 the function has local maxima.

So the corresponding maximum value is f(1)=15+51=0

At x=3 , the value of f’(x) is changing from -ve to +ve so at x=3 the function has local minima.

Now the corresponding minimum value is 

f(3)=(3)55(3)4+5(3)31=243405+1351=28.


27. A telephone company in a town has 500 subscribers on its list and collects fixed charges of Rs 300/- per subscriber per year. The company proposes to increase the annual subscription and it is believed that for every increase of Re 1/- one subscriber will discontinue the service. Find what increase will bring maximum profit? 

Ans: Let the company increase its annual subscription by x Rs.

So the x subscriber will discontinue the service.

Now the total revenue of the company after the increment

R(x)=(500x)(300+x)

R(x)=150000+500x300xx2

On differentiating both sides w.r.t. x, 

 (x)=5003002x

Now,  (x)=0

2002x=0

x=100

For finding maxima and minima of the function we again differentiate  (x)

 (x)=5003002x

  (x)=2<0

So the revenue will be maximum at x=100.

Hence the maximum value of revenue 

R(100)=(500100)(300+100)=400×400=160000

Hence the company will earn the maximum profit at the increment of 100 Rs.


28. If the straight line x cosα + y sinα = p touches the curve x2a2+y2b2=1, then prove that a2cos2α+b2sin2α=p2.

Ans: Given curvex2a2+y2b2=1

It is given that the line x cosα + y sinα = p touches the curve that means the slope of the line should be equal to the slope of the tangent to the curve.

x2a2+y2b2=1---------(1)

On differentiating both sides w.r.t. x, 

2xa2+2yb2.dydx=0

dydx=b2a2(xy)

This slope should be equal to the slope of line x cosα + y sinα = p which is slope   of   line   =coefficient   of   xcoefficient   of   y=cos   α sin   α =cot   α 

Now, b2a2(xy)=cot   α 

xa2cos α =yb2sin α 

Let k=xa2cos   α =yb2sin   α 

x=ka2cos   α   ,   y=kb2sin   α 

So the point is (ka2cos   α   ,   kb2sin   α )where the line x cosα + y sinα = p touches the curve x2a2+y2b2=1.

On substituting the point in given curve, we get 

(ka2cos α )2a2+(kb2sin α )2b2=1

k2b2a4cos2 α +k2b4a2sin2 α =a2b2

k2b2a2(a2cos2 α +b2sin2 α )=a2b2

(a2cos2 α +b2sin2 α )=1k2--------(2)

Now substituting the point (ka2cos α   ,   kb2sin α )in equation of line x cosα + y sinα = p 

ka2cos α .cos α +kb2sin α .   sin α =p

k(a2cos2 α +b2sin2 α )=p

a2cos2 α +b2sin2 α =pk

Squaring on both sides

(a2cos2 α +b2sin2 α )2=p2k2-----(3)

Now on dividing eq.(3) by eq.(2)

a2cos2 α +b2sin2 α =p2(Hence proved)


29.  An open box with square base is to be made of a given quantity of card board of area c2. Show that the maximum volume of the box is c363 cubic units.

Ans: 


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Let the side of the square base is x and its height is y.

Area of base =x2

Given: The area of the metal used is x2+4xy=c2

y=c2x24x-------(1)

Volume of the box V=x2y

On substituting the value of y from eq.1, we get

V=x2(c2x24x)

V=x4(c2x2)

V=14(c2xx3)

Now on differentiating w.r.t. x, we get

dVdx=14(c23x2)

Now, dVdx=0

14(c23x2)=0

x=±c3

Here x represents the side of the box , Hence we reject x=c3

Hence x=c3

Again, differentiating Eq.(ii) w.r.t. X, we get

d2Vdx2=3x2

d2Vdx2|x=c3=3c2<0

Therefore we get the maximum value at x=c3

So the maximum volume Vmax   =14(c2xx3)

Vmax   =14(c33c333)

Vmax   =12(c333)

Vmax   =c363cu units.


30. Find the dimensions of the rectangle of perimeter 36 cm which will sweep out a volume as large as possible, when revolved about one of its sides. Also find the maximum volume.

Ans: 


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Let the breadth and length of the rectangle 

be x and y respectively.

Perimeter of rectangle P=2x+2y

Given perimeter P=36cm

So, 2x+2y=36cm

x+y=18

y=18x------(1)

Now according to the problem, let the rectangle be revolved out about its length y , then the new shape cylinder is formed.

The volume of the cylinder is V= π x2y

On substituting y=18x from eq.1 in equation of volume , we get 

V= π x2y

V= π x2(18x)

V=18 π x2 π x3

On differentiating w.r.t. x, we get

 dVdx=36 π x3 π x2

Now dVdx=0

36 π x3 π x2=0

3 π x(12x)=0

x=0,12

Here x represents the side of a rectangle and we know that side can’t be 0. 

So, x=0(rejected)

Again, On differentiating dVdxw.r.t. x, we get

d2Vdx2= π (366x)

At x=12, d2Vdx2= π (366x)=36 π <0

Hence at x=12, Cylinder has maximum volume. 

Therefore the volume at x=12, 

V=18 π x2 π x3

=18 π (12)2 π (12)3

= π .122(1812)

=864 π   cm3


.31. If the sum of the surface areas of a cube and a sphere is constant, what is the ratio of an edge of the cube to the diameter of the sphere, when the sum of their volumes is minimum? 

Ans: Let the side of a cube is x and radius of sphere is r.

Surface area of cube =6x2

Surface area of sphere =4 π r2

And, 6x2+4 π r2=k(Given)[K=constant]

x2=k4 π r26

x=[k4 π r26]12-------(1)

Now, the volume of cube =x3

Volume of sphere =43 π r3

Le the sum  of their volume is S=x3+43 π r3

S=[k4 π r26]32+43 π r3 [Substituting the value of r from eq.1]

dSdr=32[k4 π r26]12(8 π r6)+4 π r2

dSdr=[k4 π r26]12(2 π r)+4 π r2

dSdr=   2 π r[{k4 π r26}122r]

For critical points, dSdr=0

2 π r[{k4 π r26}122r]=0

r=0   and   r=12(k4 π r26)12

r=0   and   4r2=k4 π r26

r=0   and   24r2+4 π r2=k

r=0   and   r2(24+4 π )=k

r=0   and   r2=k4(6+ π ).

r=0   and   r=12k6+ π 

re r represents the radius and radius can’t be zero. 

Hence r0.

r=12k6+ π 

On differentiating again dSdr w.r.t. r, we get

dSdr=   2 π r[{k4 π r26}122r]

 d2Sdr2=8 π r2 π [k4 π r26+r.12k4 π r26(8 π r6)

d2Sdr2=2 π [8 π r2+12(k4 π r26)12k4 π r26]+8 π r

d2Sdr2=2 π [96 π r2+72k72k4 π r26]+8 π r>0

Hence the sum of their volumes at r=12k6+ π  is minimum

At r=12k6+ π  the value of x from eq.1 is 

x=[k4 π r26]12

x=[k4 π (12k6+ π )26]12

x=[k π (k6+ π )6]12

x=[k6+ π ]12=2r--------(2)

Now the ratio of edge of cube to the diameter of sphere is =x2r

=2r2r=11

Hence the ratio of edge of cube to the diameter of sphere is 1:1


32. AB is the diameter of a circle and C is any point on the circle. Show that the area of  Δ ABC is maximum, when it is isosceles.


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Ans: 

Let the sides of the triangle is AC=x and BC=y

We have to prove that the area of the triangle is maximum and the triangle is isosceles means (x=y).

We have AB=2r, AB is the diameter of the circle

(2r)2=x2+y2

y2=4r2x2

y=4r2x2-------(1)

Now, Area of triangle A=12x.y

A=12x.(4r2x2)

Now,differentiating both sides w.r.t. x, we get

dAdx=12[4r2x2   +x.12.(4r2x2)12(2x)]

dAdx=12[4r2x2   x24r2x2   ]

dAdx=12[4r22x24r2x2]

dAdx=[2r2x24r2x2]

For critical point, dAdx=0

2r2x24r2x2=0

2r2x2=0

r=12x

x=±r2

Here x represents the side of the triangle and the side can’t be negative. So x=r2(Rejected)

For finding whether the area of triangle is maximum and minimum we again differentiate dAdx w.r.t. x, 

d2Adx2=[4r2x2(02x)2r2x224r2x2(02x)4r2x2]

d2Adx2=2x[(8r22x2)(2r2x2)2(4r2x2)32]

d2Adx2=2r[6r2x22(4r2x2)32]

At x=r2, d2Adx2=2r[4r2252r3]<0

Hence at x=r2 we get the maximum area of the triangle.

For x=r2, y=4r2(r2)2=r2

If we see, here x=y=r2 . That means the two sides of the triangle are the same. Hence the triangle is isosceles. 


33. A metal box with a square base and vertical sides is to contain 1024cm3. The material for the top and bottom costs Rs.5/cm2 and the material for the sides costs Rs 2.50/cm2. Find the least cost of the box. 

Ans: 


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Let the side of the base be x and height is y.

Volume of the box V=x2ywhich is given as 1024 .

So, x2y=1024

y=1024x2. --------(1)

The cost for bottom and top = 2x2×5and the cost for sides material= 4xy×2.50

So the total cost is denoted as C=10x2+10xy

C=10x2+10xy

 substituting the value of y from eq. 1

C=10x2+10x(1024x2)

C=10x2+10(1024x)

On differentiating w.r.t. x, 

dCdx=20x10240x2

Now, dCdx=0

20x10240x2=0

20x3=10240

x3=512

x=8

Again, differentiating dCdxw.r.t. x, we get

d2Cdx2=20+20480x3>0

Hence the cost at x=8is minimum.

The cost at x=8 is C(8)=10(8)2+10(10248)

C(8)=640+1280=1920Rs.


34.  The sum of the surface areas of a rectangular parallelopiped with sides x, 2x and  x3 and a sphere is given to be constant. Prove that the sum of their volumes is minimum, if x is equal to three times the radius of the sphere. Also find the minimum value of the sum of their volumes.

Ans: Given that we have the sides of a rectangular parallelopiped as x,   2  and   x3and the sum of the surface area of the parallelopiped and sphere is constant.

Let the sum of surface area is S

S=2(x.2x+2x.x3+x.x3)+4 π r2=k(Given)

2(x.2x+2x.x3+x.x3)+4 π r2=k

6x2+4 π r2=k

r=k6x24 π --------(1)

Volume of parallelopiped = x.2x.x3

Volume of sphere = 43 π r3 

Let the Sum of the volume of parallelopiped and volume of the sphere is V.

Then, V=x.2x.x3+43 π r3

V=x.2x.x3+43 π (k6x24 π )3

V=2x33+43 π (k6x24 π )32

V=2x33+43 π 18 π 32(k6x2)32

On differentiating w.r.t. x, we get

dVdx=2x2+43 π 18 π 3232(k6x2)12(12x)

dVdx=2x23x π (k6x2)12

Now, dVdx=0

2x23x π (k6x2)12=0

4x4=9x2 π (k6x2)

4 π x4+54x4=9kx2

x4(4 π +54)=9kx2

x2=9k4 π +54

x=3k4 π +54----------(2)

Again differentiating dVdxw.r.t. x, we get

d2Vdx2=4x3 π [(k6x2)12+x.(6x)(k6x2)12]

d2Vdx2=4x3 π [(k6x2)+(6x2)(k6x2)12]

d2Vdx2=4x3 π [(k12x2)(k6x2)12]

At x=3k4 π +54,

d2Vdx2=12k4 π +543 π [k12.9.k4 π +54k6.9.k4 π +54]

d2Vdx2=12k4 π +543 π [4 π k+54k108k4 π +544 π k+54k6.9.k4 π +54]

d2Vdx2=12k4 π +543 π [4 π k54k4 π +544 π k4 π +54]

d2Vdx2=12k4 π +546k π [2 π 274 π +544 π k] since (2 π 27)0d2Vdx20;k>0

For x=3k4 π +54, the sum of volumes is minimum.

From eq. 1 , for x=3k4 π +54

r=k6x24 π 

r=k6.9(k4 π +54)4 π 

r=12 π 4 π k+54k54k4 π +54

r=k4 π +54=x3

x=3r

Hence x is 3 times the radius of the sphere.

Now the minimum sum of volume is

V|x=3k4 π +54=2x33+43 π r3

=2x33+43 π (x3)3=18.(k4 π +54)32(1+2 π 27)


Choose the correct answer from the given four options in each of the following questions 35 to 39:

35. The sides of an equilateral triangle are increasing at the rate of 2 cm/sec. The rate at which the area increases, when side is 10 cm is: 

(A). 10cm2/s

(B). 3cm2/s

(C). 103cm2/s

(D). 103cm2/s

Ans: Let the side of the equilateral triangle is xcm.

Area of equilateral triangleA=34x2------(1)

Given that dxdt=2cm/sec

On differentiating the eq.(1). W.r.t. t, we get

dAdt=34.2x.dxdt

When x=10cm   and dxdt=2cm/sec

dAdt=34.40=103cm2/sec

Hence the area is increasing at the rate of 103cm2/sec.


36. A ladder, 5 meter long, standing on a horizontal floor, leans against a vertical wall. If the top of the ladder slides downwards at the rate of 10 cm/sec, then the rate at which the angle between the floor and the ladder is decreasing when lower end of ladder is 2 metres from the wall is:

 (A). 110radian/sec 

 (B). 120radian/sec

 (C). 20 radian/sec

 (D). 10 radian/sec

Ans: 


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Let the angle b/w floor and the ladder is  θ 

And sides of the triangle are x,   ywhere BC=  &   AB=x

From the figure,

sin θ =x500         and   cos θ =y500 

x=500sin θ     and   y=500cos θ -----(1)

Given that the top of the ladder slides downwards at the rate of 10cm/sec.

Hence dxdt=10cm/sec

x=500sin θ 

Differentiating on both sides w.r.t. T, we get

dxdt=500cos θ .θ dt

θ dt=10500cos θ 

On substituting the value of cos θ =y500from the eq.1

θ dt=10500.y500

θ dt=10y

Given that lower end from the wall is 2m=200cm

Hence, θ dt=10200rad/sec

θ dt=120rad/sec

Therefore option B is the correct answer.


37. The curve y=x15 has at (0, 0) 

(A) a vertical tangent (parallel to y-axis) 

(B) a horizontal tangent (parallel to x-axis) 

(C) an oblique tangent 

(D) no tangent

Ans: The curve we have y=x15

Now the slope of the tangent on the curve is

dydx=15(x)45

Slope on point (0,0) is

dydx|(0,0)=15(x)45=15(0)45=

Here slope of the tangent is and we know that    is the slope of the vertical line. So the curve y=x15has a vertical tangent at (0,0) which is parallel to y-axis. Therefore option A is the correct option.


38.  The equation of normal to the curve 3x2y2=8 which is parallel to the line x + 3y = 8 is :

(A) 3x – y = 8 

(B) 3x + y + 8 = 0 

(C) x + 3y ± 8 = 0 

(D) x + 3y = 0

Ans: The given curve we have 3x2y2=8and the equation of the line is x + 3y = 8.

x+3y=8

y=13x+83

On comparison with equation y=mx+  swe get the slope as m=13. This slope should be equal to the slope of normal as it is given that the line is parallel to the normal and we know that parallel lines have the same slope. 

Now, 3x2y2=8

On differentiating w.r.t. x , we get 6x2y.dydx=0

dydx=3xy= slope of the curve

Now, Slope of the normal to the curve=1slope   of   the   curve=1(dydx)=y3x

y3x=13

y=x------(1)

On substituting  this value in the given equation of thecurve, we get

3x2x2=8

x2=4

x=±2

For x=2, from eq.1 ,y=x=2

For x=2, x=2

So the points are (2,2) &(-2,-2) at which normal is parallel to line x + 3y = 8 .

Now the equation of normals passing through points (2,2) &(-2,-2) and having slope 13.

(y2)=13(x2)     and   (y+2)=13(x+2)

3(y2)+(x2)=0     and   3(y+2)+(x+2)=0

3y+x8=0     and   3y+x+8=0

Hence the equation of normals is 3y+x±8=0   . So option B is the correct option.


39. If the curve ay+x2=7 and x3=y, cut orthogonally at (1, 1), then the value of a is: 

(A).  1 

(B).  0 

(C). – 6 

(D). 6

Ans: Given that the curves cut orthogonally at point (1,1) that means the slope of both the curves should satisfy the condition m1.m2=1where let m1is the slope of curve ay+x2=7 and m2is the slope of curvex3=y.

ay+x2=7

Differentiating on both sides w.r.t. x, we get

adydx+2x=0

dydx=2xa

dydx|(1,1)=2xa=2a=m1(Say)

Now, x3=y

Differentiating on both sides w.r.t. x, 

3x2=dydx

dydx|(1,1)=3(1)=3=m2(Say)

Given that both curves cut orthogonally. So, 

m1.m2=1

2a.3=1

a=6

Therefore the correct option is D.


40.  If y=x410 and if x changes from 2 to 1.99, what is the change in y? 

(A) 0.32 

(B) 0.032 

(C) 5.68 

(D) 5.968

Ans: We have the curve y=x410

We know that dydx=y+ Δ yy Δ x

 Δ y=dydx. Δ x

Now, y=x410

On differentiating on both sides w.r.t. x, we get

dydx=4x3

Changes in x is  Δ x=21.99=0.01

Now,  Δ y=dydx. Δ x

 Δ y=4(2)3×(0.01)

 Δ y=0.32

Hence option A is the correct answer.


41. The equation of tangent to the curve y(1+x2)=2x, where it crosses x-axis is: 

(A) x + 5y = 2 

(B) x – 5y = 2 

(C) 5x – y = 2 

(D) 5x + y = 2

Ans: Given curve y(1+x2)=2x

The curve passes crosses the x-axis, hence y=0

0(1+x2)=2x

2x=0

x=2

Hence the point where the tangent passes through is (2,0).

Now, we find the slope of the tangent on curve y(1+x2)=2x

On differentiating both sides w.r.t. x, we get 

2xy+(1+x2)dydx=1

dydx=12xy1+x2

Slope at the point (2,0)

dydx|(2,0)=12xy1+x2=15

Now the equation of tangent passing through (2,0) and having slope 15

(y0)=15(x2)

5(y0)=1(x2)

x+5y=2

Hence the correct option is option A.


42.  The points at which the tangents to the curve y=x312x+18are parallel to x-axis are: 

(A) (2, –2), (–2, –34) 

(B) (2, 34), (–2, 0) 

(C) (0, 34), (–2, 0) 

(D) (2, 2), (–2, 34)

Ans: The given curve is y=x312x+18

For finding the slope of tangent on the curve we differentiate the given equation of curve w.r.t. x, 

dydx=3x212

Given that the tangent is parallel to the x-axis, so the above slope will be equal to 0 as we know that the slope of the x-axis is 0 and if two lines are parallel then their slopes are to be equal. Therefore,

dydx=0 

3x212=0

x2=4

x=±2

For x=2, y=(2)324+18=2

For x=2, y=(2)3+24+18=34

Hence the required points are (2,2) and (-2,34) where the tangents to the curve  y=x312x+18 are parallel to x-axis.


43.  The tangent to the curve y=e2x at the point (0, 1) meets x-axis at:

 (A) (0, 1) 

(B) (12,0)       

(C) (2, 0) 

(D) (0, 2)

Ans: The given curve we have y=e2x

Slope of tangent to the curve is

dydx=2.e2x

Slope at point dydx|(0,1)=2.e0=2

Now, the equation of a tangent passing through the point (0,1) and having slope 2 is:

(y1)=2(x0)

2xy+1=0

It is asked that the tangent meets x-axis , so on substituting y=0 in equation of tangent, we get

2x(0)+1=0

x=12

Thus the point where tangent meets the x-axis is (12,0). So option B is the correct answer.


44. The slope of tangent to the curve x=t2+3t8, y=2t22t5at the point (2, –1) is: 

(A) 227 

(B) 67 

(C) 67 

(D) – 6

Ans: The equation of curve we have

 x=t2+3t8   and     y=2t22t5

Differentiating w.r.t. t,

dxdt=2t+3   and     dydt=4t2

We know that the slope is dydx=dydtdxdt

So dydx=4t22t+3

Given that the curve passes through the point (2, –1) , So

2=t2+3t8   and     1=2t22t5

t2+3t10=0   and     2t22t4=0

t2+5t2t10=0   and     2t24t+2t4=0

t(t+5)2(t+5)=0   and     2t(t2)+2(t2)=0

(t+5)(t2)=0   and     (t2)(2t+2)=0

t=5,2   and     t=1,2

Here the common value of t is 2.

Now at t=2 the slope of tangent is 

dydx|t=2=4(2)22(2)+3=67

Therefore the slope of tangent to the curve is 67. Hence option B is the correct answer.


45.  The two curves x33xy2+2=0and 3x2yy32=0 intersect at an angle of 

(A)  π 4 

(B)    π 3

(C)  π 2

(D)  π 6

Ans: The equations of curves we have 

x33xy2+2=0   and   3x2yy32=0

On Differentiating the first curve w.r.t. x, we get 

3x23y23x.2y.dydx=0

dydx=3x23y26xy=m1(Say)

Now On differentiating the second curve 3x2yy32=0, we get

6xy+3x2dydx3y2dydx=0

dydx=6xy3y23x2=m2(Say)

If we see m1.m2=3x23y26xy×6xy3y23x2=1

We know that if m1.m2=1then both the curves intersect at right angle i.e. making angle  π 2with each other.  


46.  The interval on which the function f(x)=2x3+9x2+12x1 is decreasing is: 

(A) [–1, ∞ ) 

(B) [–2, –1] 

(C) (– ∞ , –2] 

(D) [–1, 1]

Ans: We have the function f(x)=2x3+9x2+12x1

 (x)=6x2+18x+12

=6(x2+3x+2)

=6(x2+2x+x+2)

=6(x+2)(x+1)

We know that a function is said to be decreasing if  (x)0

So on drawing the points on number line 


seo images


We see that  (x)<0in interval [-2,-1]. Hence the given function is decreasing in [-2,-1], So option B is the correct answer.


47. Let the f : R → R be defined by f(x)=2x+cosx, then f : 

(A) has a minimum at x = π 

(B) has a maximum, at x = 0 

(C) is a decreasing function 

(D) is an increasing function

Ans: The given function is f(x)=2x+cosx

Differentiating the function w.r.t. x, 

 (x)=2sinx

We know that  1sinx1

So, 1+22sinx1+2

12sinx3

1 (x)3

Since  (x)>0

Hence the function is an increasing function. Option D is the correct answer.


48. y=x(x3)2 decreases for the values of x given by : 

(A) 1 < x < 3 

(B) x < 0 

(C) x > 0 

(D) 0<x<32

Ans: The given function is y=x(x3)2

Differentiating the given function w.r.t. x,

dydx=(x3)2+x.2(x3)

=x26x+9+2x26x

=3x212x+9

We know that the function is decreasing function if  (x)<0

3x212x+9<0

x24x+3<0

x2x3x+3<0

(x3)(x1)<0


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So the function decreases for (1,3). Hence option A is the correct answer.


49. The function f(x)=4sin3x6sin2x+12sinx+100 is strictly 

(A) increasing in ( π ,   π 2)       

(B) decreasing in ( π 2π )    

(C) decreasing in [π 2, π 2]  

(D) decreasing in [0, π 2]

Ans: The given function is f(x)=4sin3x6sin2x+12sinx+100

Differentiating the given function w.r.t. x, 

 (x)=12sin2x.cosx12sinx.cosx+12cosx

 (x)=12cosx[sin2xsinx+1]

 (x)=12cosx[sin2x+(1sinx)]

We know that sin2x0and 1sinx101sinx2

sin2x+1sinx0

Hence  (x)<0if cosx<0   and   we   know   that   cosx<0   in   ( π 2, π )   

So the function is decreasing function in ( π 2, π ). Hence option B is the correct  answer.


50. Which of the following functions is decreasing on (0,π2)      

(A) sin2x 

(B) tanx 

(C) cosx 

(D) cos 3x

Ans: We know that a function is said to be decreasing function if  (x)<0   in a given domain. Here the given domain is  (0, π 2). If we see the derivative of cosx is sinx   which is sinx<0   in   (0, π 2).   

So f(x)=cosx

Differentiating the function w.r.t. x, 

 (x)=sinx

sinx<0   in   (0, π 2)

Hence option C is the correct answer.


51. The function f(x)=tanxx 

(A) always increases 

(B) always decreases 

(C) never increases 

(D) sometimes increases and sometimes decreases.

Ans: The given function is f(x)=tanxx

Differentiating the function w.r.t. x, 

 (x)=sec2x1

secxR(1,1)

sec2x1>0

 (x)>0,xR

Hence tanxx always an increasing function. So option A is the correct answer.


52.  If x is real, the minimum value of x28x+17 is 

(A) –1 

(B) 0 

(C) 1 

(D) 2

Ans: The given function is f(x)   =   x28x+17

Differentiating the function w.r.t. x,

 (x)   =   2x8

For critical point  (x)=0

2x8=0

x=4

We know that a function has local minima if f’’(x)>0. 

So here we have to find the 2nd derivative of the given function.

 (x)   =   2x8

  (x)   =   2>0   

Hence  we get the minimum value at x=4. 

Now the value of function at x=4 is the minimum value of the function.

f(4)=   (4)28(4)+17=1632+17=1

Hence the option c is the correct answer.


53.  The smallest value of the polynomial x318x2+96x in [0, 9] is 

(A) 126 

(B) 0 

(C) 135 

(D) 160

Ans: The given polynomial function is f(x)=x318x2+96x

Differentiating the function w.r.t. x, 

 (x)=3x236x+96

 (x)=3x236x+96

So,  (x)=0

3x236x+96=0

x212x+32=0

x28x4x+16=0

(x8)(x4)=0

x=4,8[0,9]

Now we find the value of given function at points x=0,4,8,9 

f(0)=(0)318(0)2+96(0)=0

f(4)=(4)318(4)2+96(4)=64288+384=160

f(8)=(8)318(8)2+96(8)=5121152+768=128

f(9)=(9)318(9)2+96(9)=7291458+864=135

If we see the minimum absolute value of the function is 0 in [0,9]. Hence the correct option is option B.


54.  The function f(x)=2x33x212x+4, has 

(A) two points of local maximum 

(B) two points of local minimum 

(C) one maxima and one minima 

(D) no maxima or minima

Ans: The given function is f(x)=2x33x212x+4

Differentiating the function w.r.t. x,

 (x)=6x26x12

For critical points  (x)=0

6x26x12=0

x2x2=0

x22x+x2=0

x(x2)+1(x2)=0

x=2,1

Differentiate again  (x)w.r.t. x,

  (x)=12x6

At x=-1,   (1)=12(1)6=18<0. Hence the function has local maxima at x=-1.

At x=2,   (2)=12(2)6=18>0. Hence the function has local minima at x=2.

Since the function has one maxima and one minima. So the option C is the correct answer.


55. The maximum value of sin x . cos x is 

(A) 14

(B) 12 

(C) 2

(D) 22

Ans: We have the function f(x)=sinx.cosx=12.2sinx.cosx=12sin2x 

 Differentiating the function w.r.t. x,

 (x)=12cos2x.2=cos2x

Now,  (x)=0

cos2x=0

2x= π 2

x= π 4

Again differentiating  (x)w.r.t. x,

  (x)=12cos2x.2=2sin2x

[  (x)]at   x= π 4=2sin( π 2)=2<0. Hence at x= π 4, the function has local maxima.

The value of the function at x= π 4

f(x)=sinx.cosx=sin π 4.cos π 4=12.12=12. So option B is the correct answer.


56.  At x=5π6, f(x)=2sin3x+3cos3xis: 

(A) maximum 

(B) minimum 

(C) zero 

(D) neither maximum nor minimum.

Ans: The function we have, is f(x)=2sin3x+3cos3x

Differentiating the function w.r.t. x,

 (x)=6cos3x9sin3x

For finding, the function has minima or maxima at x=5 π 6, we again differentiate  (x)w.r.t. x, 

  (x)=18sin3x27cos3x=9(2sin3x+3cos3x)

Now, [ (x)]x=5 π 6=6cos(3.5 π 6)9sin(3.5 π 6)

=6cos(5 π 2)9sin(5 π 2)

=6cos(2 π + π 2)9sin(2 π + π 2)=09=9

At x=5 π 6,  (x)0 So x=5 π 6is neither point of maxima or minima. Hence option D is the correct answer.


57. Maximum slope of the curve y=x3+3x2+9x27is: 

(A) 0 

(B) 12 

(C) 16 

(D) 32

Ans: The curve we have, y=x3+3x2+9x27

Slope of the curve = dydx=3x2+6x+9

Now we find whether the slope of the curve is maximum or minimum.

Differentiating the slope of the curve w.r.t. X,

d2ydx2=6x+6=6(x1)

For critical points, d2ydx2=0 

6(x1)=0

x=1

Now, d3ydx3=6<0. Hence we get the maximum slope at x=1.

So the slope of curve at x=1,

 dydx|x=1=3(1)2+6(1)+9=153=12

Therefore the correct answer is option B.


58. f(x)=xx has a stationary point at 

(A) x=e 

(B) x=1e 

(C) x=1 

(D) x=e

Ans: The function we have, f(x)=xx

Let y=xx

Taking log on both sides

logy=x.log(x)

Differentiating on both sides w.r.t. x,

1y.dydx=log(x)+x.1x

dydx=y.[1+logx]

dydx=xx.[1+logx]

For the stationary points dydx=0

xx.[1+logx]=0       [x  always +ve]

1+log   x=0

log   x=1

log   x=loge1

   x=e1

   x=1e

This is the stationary point. So option B is the correct answer.


59. The maximum value of  (1x)xis: 

(A) e 

(B) ee

(C) e1e 

(D) (1e)1e   

Ans: The function we have, f(x)=(1x)x

Let y=(1x)x

Taking log on both sides 

logy=x.log(1x)

Differentiating on both sides w.r.t. x,

1y.dydx=log(1x)+x.11x(1x2)

dydx=y[log(1x)1]

dydx=(1x)x[log(1x)1]

For critical points, dydx=0

(1x)x[log(1x)1]=0

log(1x)=1   {(1x)x is always +ve}

log(1x)=loge

1x=e

x=1e

Again differentiating dydxw.r.t. x, we get:

dydx=(1x)x.log(1x)(1x)x

d2ydx2=(1x)x.(1x)+[log(1x)1]2(1x)x

Now, d2ydx2|x=1e=(e)1e(e)+(11)2(e)1e=e.(e)1e<0

Hence the function has maxima at x=1e

So the maximum value of function at x=1e

f(1e)=(11e)1e=(e)1e

Therefore option C is the correct answer.


Fill in the blanks in each of the following Exercises 60 to 64:

60. The curves y=4x2+2x8 and y=x3x+13 touch each other at the point_____. 

Ans: The given curves are y=4x2+2x8and y=x3x+13

Given that the curves touch each other so the slope of both the curves should be the same.

For slope of curves, differentiate both curves w.r.t. x, 

y=x3x+13

Differentiating w.r.t. x,

dydx=3x21-----(1)

Also, y=4x2+2x8

Differentiating w.r.t. X,

dydx=8x+2------(2)

Now equating the slopes from eq.1 and eq.2

3x21=8x+2

3x28x3=0

3x29x+x3=0

3x(x3)+1(x3)=0

(3x+1)(x3)=0

x=3,13

For x=3,

   y=4(3)2+2(3)8=36+68=34

For x=13 

   y=4(13)2+2(13)8=49238=749

So the required points are (3,34)and (13,749).


61. The equation of normal to the curve y=tanx at (0, 0) is ________.

Ans: The given curve is y=tanx

Differentiating the curve w.r.t. x,

dydx=sec2x

Now the slope of the curve at (0,0)

dydx|(0,0)=sec2x=1

Slope of Normal=1slope   of   curve=11=1

The equation of normal passing through (0,0) and having slope -1

(y0)=1(x0)

y+x=0

So the equation of normal to the curve y=tanx at (0, 0) is y+x=0.


62. The values of a for which the function f(x)=sin xax+b increases on R are ______.

Ans: The given function is f(x)=sin   xax+b

Differentiating the function w.r.t. x, 

 (x)=cos   xa

We know that a function is increasing function if  (x)>0   in its domain.

Given that the function is increasing in R.

Hence  (x)>0

cos   xa>0

cos   x>a

Since cos   x[1,1]

For the condition cos   x>a being true the minimum value of cosx should be greater than a i.e. 1>a

So the values of a should be (,1)for which the function f(x)=sin   xax+b increases on R.


63. The function f(x)=2x21x4,x>0, decreases in the interval _______.

Ans: The given function is f(x)=2x21x4

Differentiating the function w.r.t. x, 

 (x)=4x.x4(2x21).4x3x8

 (x)=4x5+4x3x8=4x3(x2+1)x8

A function is decreasing function if  (x)<0

4x3(x2+1)x8<0

x2+1<0        [Given that x>0, so x8>0   and   4x3>0]

 x21>0

(x1)(x+1)>0

x>1,x<1

x>0(given)

   x<1(Rejected)

So the function decreases in interval (1,).


64. The least value of the function f(x)=ax+bx (a>0,b>0,x>0)is ______.

Ans: The given function is f(x)=ax+bx

Differentiating the function w.r.t x,

 (x)=abx2

For critical points,  (x)=0

abx2=0

x2=ba

x=±ba

Since given that x>0, hence x=ba(Rejected)

Now again differentiating  (x)   w.r.t. x,

  (x)=b.2x3=2bx3

At x=ba,   (x)|x=ba=2b(ba)32=2a32b12

Since a>0 and b>0 , Hence   (x)>0.therefore the function has minima at x=ba.

Now the minimum value at x=ba

f(ba)=a.ba+bba

f((ba)=ab+ab=2ab

Therefore the least value is 2ab...


Chapter 12 - Application of Derivatives

Chapter 12 - Application of Derivatives is an important Chapter for the boards of Class 12th. The Exemplar will help students to solve the questions and strengthen important concepts and will act as a revision. Students will learn the concept of rate of change of quantitative, tangents and normal approximation, increasing and decreasing functions, maximum and minimum values of functions in closed intervals, and maxima and minima through Chapter 6. Students focusing on the strong grip on the Chapter can reach out the Vedantu’s official site and go for the exemplar for the respective Chapter. 

 

Vedantu also offers other online material for Chapter 6 - Application of Derivatives like notes, sample question papers, additional questions, and last year’s papers. The notes offered by the Vedantu are designed by their expert faculty for the students to understand the theoretical concepts of the Chapter. A good collection of sample papers and additional questions are also offered that will give an upper hand to students for the practice during the preparation. The PDF also consists of last year’s paper so that the students can know what type of questions are asked in the Exam. Vedantu also offers live sessions for the students, so that they can revise the topics they have learned and clear doubts from the teachers. The teaching faculty of Vedantu is a group that focuses on enhancing the results for the registered students.

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FAQs on NCERT Exemplar for Class 12 Maths - Application Of Derivatives - Free PDF Download

1. From where do students download the exemplar for the Chapter 6 Application of Derivatives?

Students can easily visit Vedantu’s official website and can find the exemplar PDF for the Chapter 6 Application of Derivatives.  The NCERT for the Chapter 6 Application of Derivatives exemplar is available in PDF form. The PDF is designed as per the latest syllabus advised by the Central Board of Secondary Education. The PDF is designed by the expert faculty of Vedantu as per the latest syllabus students following the Exemplar will have an upper hand because of the content mentioned in the PDF.

2. How is Vedantu beneficial for the students in their studies?

Vedantu is very beneficial when it comes to studying as Vedantu offers online material like notes, sample question papers, additional questions, and last year’s papers. A good collection of sample papers and additional questions are also offered that will give an upper hand to students for the practice during the preparation. The notes offered by the Vedantu are designed by the expert faculty of Vedantu for the students to understand the theoretical concepts of the respective Chapter. 

3. Is Chapter 6 Application of Derivatives important for Class 12?

Chapter 6 Application of Derivatives is very important for Class 12 as the Chapter consists of many important topics and concepts that are surely asked in the Exams. Students will learn the concept of rate of change of quantitative, tangents and normal approximation, increasing and decreasing functions, maximum and minimum values of functions in closed intervals, and maxima and minima.

4. How is the Exemplar of Chapter 6 Application of Derivatives?

The Exemplar for the Chapter 6 Application of Derivatives is designed as per the latest syllabus advised by the Central Board of Secondary Education. The Exemplar is designed by the expert faculty of Vedantu itself. The Exemplar consists of many important topics that are constantly asked in different ways. The topics have additional questions that can be used for practice in the preparation period.