Courses
Courses for Kids
Free study material
Offline Centres
More
Store Icon
Store
seo-qna
SearchIcon
banner

Find the value of the following. cos72

Answer
VerifiedVerified
440.1k+ views
like imagedislike image
Hint: Since it is not possible to find the angles of the trigonometric function individually, try to bring those angles in the form of the angles which are easily available to us or which we can find out easily.

Complete step-by-step answer:
We have to find the value of a trigonometric function of the angle 72 . Since we directly do not know the value of the given angle. We will try to use some already known trigonometric identities to find the answer of the question given.
We know that cos(π2x)=sinx . So, we will use this identity to represent cos(72) in terms of sin . We observe that, cos72=cos(9018)
On using the identity cos(π2x)=sinx we get,
 cos72=sin18
So, we will try to find the value of sin18 . Let us take A to be 18 , that is A=18 .
Then we can see that 5A=90 .
 2A+3A=90
 2A=903A
Now on applying sine function to the angles on both sides of the equation we get,
 sin2A=sin(903A)
Now again by using the identity cos(π2x)=sinx , we get
 sin2A=cos3A
Now we will use the identities for multiple angles to get,
 2sinAcosA=4cos3A3cosA [Using sin2x=2sinxcosx and cos3x=4cos3x3cosx ]
 2sinAcosA4cos3A+3cosA=0
 cosA(2sinA4cos2A+3)=0
Since cosA=cos180 , we have
 2sinA4cos2A+3=0
 2sinA4(1sin2A)+3=0 [Using cos2x+sin2x=1 ]
 2sinA4+4sin2A+3=0
 2sinA+4sin2A1=0 , which is a quadratic equation in sinA . So, we use the quadratic formula x=b±b24ac2a given for the equation ax2+bx+c=0 .
Therefore, sinA=2±44(4)(1)2(4)=2±4+168
 sinA=2±258
 sinA=1±54
Since 18 belongs to the first quadrant, we know that sin18 is positive.
Therefore, sin18=514 .
So, we have cos72=sin18=514 .
Hence the value of cos72 is 514 .
So, the correct answer is “ 514 ”.

Note: While solving this kind of problem, one should remember that three are many ways of getting to the answer using various trigonometric identities. But we have to be clear with what we need to find and use a relatively easier method. Remember to mention the identities used wherever required. Such as sin2x=2sinxcosx etc.