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JEE Main
Exam
JEE Main
Subject
Mathematics
Chapter
Differentiation

JEE Main · Mathematics · Differentiation

Let f(x) be a bijective differentiable function and h(x) be its...

Let f(x)f(x) be a bijective differentiable function and h(x)h(x) be its inverse function. It is given that f(1)=2f(1) = 2, f(3)=9f(3) = 9, f′(1)=3f^{\prime}(1) = 3, and f′(3)=12f^{\prime}(3) = \dfrac{1}{2}. If a function g(x)g(x) is defined as g(x)=(h(x))2⋅h(x3+1)g(x) = (h(x))^2 \cdot h(x^3 + 1), then the value of g′(2)g^{\prime}(2) is equal to:

Options

  1. A24
  2. B26
  3. C4
  4. D25

Correct answer

B. 26

Step-by-step solution

Since h(x)h(x) is the inverse of f(x)f(x), we have f(h(x))=xf(h(x)) = x.
From the given values:
f(1)=2  ⟹  h(2)=1f(1) = 2 \implies h(2) = 1
f(3)=9  ⟹  h(9)=3f(3) = 9 \implies h(9) = 3

Using the derivative of an inverse function, h′(x)=1f′(h(x))h^{\prime}(x) = \dfrac{1}{f^{\prime}(h(x))}.
At x=2x = 2: h′(2)=1f′(h(2))=1f′(1)=13h^{\prime}(2) = \dfrac{1}{f^{\prime}(h(2))} = \dfrac{1}{f^{\prime}(1)} = \dfrac{1}{3}
At x=9x = 9: h′(9)=1f′(h(9))=1f′(3)=11/2=2h^{\prime}(9) = \dfrac{1}{f^{\prime}(h(9))} = \dfrac{1}{f^{\prime}(3)} = \dfrac{1}{1/2} = 2

Now, differentiate g(x)=(h(x))2⋅h(x3+1)g(x) = (h(x))^2 \cdot h(x^3 + 1) using the product rule and chain rule:
g′(x)=[2h(x)h′(x)]h(x3+1)+(h(x))2[h′(x3+1)⋅3x2]g^{\prime}(x) = \left[ 2h(x)h^{\prime}(x) \right] h(x^3 + 1) + (h(x))^2 \left[ h^{\prime}(x^3 + 1) \cdot 3x^2 \right]

Substitute x=2x = 2 into the derivative:
g′(2)=2h(2)h′(2)h(9)+(h(2))2h′(9)⋅3(2)2g^{\prime}(2) = 2 h(2) h^{\prime}(2) h(9) + (h(2))^2 h^{\prime}(9) \cdot 3(2)^2

Substitute the known values:
g′(2)=2(1)(13)(3)+(1)2(2)⋅12g^{\prime}(2) = 2 (1) \left(\dfrac{1}{3}\right) (3) + (1)^2 (2) \cdot 12
g′(2)=2+24=26g^{\prime}(2) = 2 + 24 = 26

Answer: 26
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