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JEE Main 2023
Differentiation
Differentiation
Easy

Question

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 :

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Solution

Given the function composition f(g(x)) is the identity function, it means f(g(x)) = x for all x. \Rightarrow ƒ'(g(x)) g'(x) = 1 put x = a \Rightarrow ƒ'(b) g'(a) = 1 \Rightarrow ƒ'(b) = 15{1 \over 5}

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