
If , and is its inverse function, then equals
A.
B.
C.
D.
Answer
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Hint: Here we will use the concept that inverse of a function gives the value on which we are applying the function, i.e. and then find the differentiation of g with respect to x. Substituting the value of as 7 we calculate the value of x and substitute in the differentiation of g.
* Chain rule of differentiation is given as where is differentiation of f and is differentiation of g with respect to x.
Complete step-by-step answer:
We have the function
We find the differentiation of
… (1)
Now we know that is an inverse function of .
Then we can write that when g is applied to f we get the value x
Now we differentiate both sides of the equation with respect to x
Using chain rule on LHS of the equation and differentiating RHS of the equation.
Dividing both sides of the equation by
Cancel the same terms from numerator and denominator.
Substituting the value from equation (1), we get
… (2)
We know that , We have to find the value of . We find the value of , i.e.
Substitute the value of
Shift all values to one side of the equation
Now we factorize the equation by writing
Take x common from first two terms and 1 common from last two terms
Pair up the factors
Now we can equate both the factors equal to zero.
Firstly
Shift the constant to one side of the equation.
Secondly,
Shift the constant to one side of the equation.
So,
But we have a condition i.e.
So we have to reject the value of .
So, our value of .
Now we substitute the value in equation (2)
So, the value of
So, option C is correct.
Note: Students should always keep in mind that the sign changes from negative to positive and vice versa when shifting any value from one side of the equation to another. Also, while applying chain rule of differentiation, always keep in mind that we differentiate the outer function first without looking at the function in the bracket then we differentiate the function inside the bracket.
Students many times make mistake of writing inverse of function by taking the reciprocal of the function, i.e. if , then they write which is wrong.
* Chain rule of differentiation is given as
Complete step-by-step answer:
We have the function
We find the differentiation of
Now we know that
Then we can write that when g is applied to f we get the value x
Now we differentiate both sides of the equation with respect to x
Using chain rule on LHS of the equation and differentiating RHS of the equation.
Dividing both sides of the equation by
Cancel the same terms from numerator and denominator.
Substituting the value from equation (1), we get
We know that
Substitute the value of
Shift all values to one side of the equation
Now we factorize the equation by writing
Take x common from first two terms and 1 common from last two terms
Pair up the factors
Now we can equate both the factors equal to zero.
Firstly
Shift the constant to one side of the equation.
Secondly,
Shift the constant to one side of the equation.
So,
But we have a condition
So we have to reject the value of
So, our value of
Now we substitute the value in equation (2)
So, the value of
So, option C is correct.
Note: Students should always keep in mind that the sign changes from negative to positive and vice versa when shifting any value from one side of the equation to another. Also, while applying chain rule of differentiation, always keep in mind that we differentiate the outer function first without looking at the function in the bracket then we differentiate the function inside the bracket.
Students many times make mistake of writing inverse of function by taking the reciprocal of the function, i.e. if
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