Medium MCQ +4 / -1 PYQ · JEE Mains 2020

Let ƒ and g be differentiable functions on R such that fog is the identity function. If for some a, b $\in$ R, g'(a) = 5 and g(a) = b, then ƒ'(b) is equal to :

  1. A 1
  2. B 5
  3. C ${2 \over 5}$
  4. D ${1 \over 5}$ Correct answer

Solution

Given the function composition f(g(x)) is the identity function, it means f(g(x)) = x for all x. <br><br>$\Rightarrow$ ƒ'(g(x)) g'(x) = 1 <br><br>put x = a <br><br>$\Rightarrow$ ƒ'(b) g'(a) = 1 <br><br>$\Rightarrow$ ƒ'(b) = ${1 \over 5}$

About this question

Subject: Mathematics · Chapter: Differentiation · Topic: Chain Rule

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