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Question: Answered & Verified by Expert
If $f(x)=\frac{1}{1+\frac{1}{x}}$ and $g(x)=\frac{1}{1+\frac{1}{f(x)}}$, then $g^{\prime}(2)$ is equal to
MathematicsDifferentiationTS EAMCETTS EAMCET 2013
Options:
  • A $\frac{1}{5}$
  • B $\frac{1}{25}$
  • C $5$
  • D $\frac{1}{16}$
Solution:
1921 Upvotes Verified Answer
The correct answer is: $\frac{1}{25}$
$\begin{aligned} & \quad f(x)=\frac{1}{1+\frac{1}{x}}=\frac{x}{1+x} \\ & \text { and } \quad g(x)=\frac{1}{1+\frac{1}{f(x)}}=\frac{1}{1+\frac{1+x}{x}}=\frac{x}{2 x+1} \\ & \therefore \quad g^{\prime}(x)=\frac{(2 x+1) \cdot 1-x(2)}{(2 x+1)^2}=\frac{1}{(2 x+1)^2} \\ & \text { Now, } g^{\prime}(2)=\frac{1}{(4+1)^2}=\frac{1}{25}\end{aligned}$

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