JEE Advance - Mathematics (2025 - Paper 2 Online - No. 14)
Let $\mathbb{R}$ denote the set of all real numbers. Let $f: \mathbb{R} \rightarrow \mathbb{R}$ and $g: \mathbb{R} \rightarrow(0,4)$ be functions defined by
$$ f(x)=\log _e\left(x^2+2 x+4\right), \text { and } g(x)=\frac{4}{1+e^{-2 x}} $$
Define the composite function $f \circ g^{-1}$ by $\left(f \circ g^{-1}\right)(x)=f\left(g^{-1}(x)\right)$, where $g^{-1}$ is the inverse of the function $g$.
Then the value of the derivative of the composite function $f \circ g^{-1}$ at $x=2$ is ________________.
Explanation
Function $ f $:
$ f(x) = \log_e\left(x^2 + 2x + 4\right) = \log_e\left[(x+1)^2 + 3\right] $
Function $ g $:
$ g(x) = \frac{4}{1 + e^{-2x}} $
To find the inverse of $ g $, $ g^{-1} $, we solve for $ x $:
Start with $ y = \frac{4}{1 + e^{-2x}} $.
Rearranging gives $ 1 + e^{-2x} = \frac{4}{y} $.
Solving for $ e^{-2x} $, we have $ e^{-2x} = \frac{4}{y} - 1 $.
Taking the natural logarithm, we find $ -2x = \ln\left(\frac{4}{y} - 1\right) $.
Thus, we solve for $ x $:
$ x = -\frac{1}{2} \ln\left(\frac{4-y}{y}\right) = \frac{1}{2} \ln\left(\frac{y}{4-y}\right) $
So, the inverse function:
$ g^{-1}(x) = \frac{1}{2} \ln\left(\frac{x}{4-x}\right) $
Now, let's evaluate $ g^{-1}(2) $:
$ g^{-1}(2) = \frac{1}{2} \ln\left(\frac{2}{4-2}\right) = \frac{1}{2} \ln(1) = 0 $
Thus, $ f(g^{-1}(x)) = f\left(\frac{1}{2} \ln\left(\frac{x}{4-x}\right)\right) $.
Next, we differentiate $ f \circ g^{-1} $:
$ \frac{d}{dx}\left(f(g^{-1}(x))\right) = \frac{d}{dx}\left[\log_e\left(\left(g^{-1}(x) + 1\right)^2 + 3\right)\right] $
Using the chain rule, the derivative is:
$ \frac{1}{\left[\left(g^{-1}(x) + 1\right)^2 + 3\right]} \cdot 2 \left(g^{-1}(x) + 1\right) \cdot \frac{d}{dx}\left(g^{-1}(x)\right) $
Calculate the derivative of $ g^{-1}(x) $:
$ \frac{d}{dx}\left(\frac{1}{2} \ln\left(\frac{x}{4-x}\right)\right) = \frac{1}{2} \cdot \frac{1}{\frac{x}{4-x}} \cdot \left(\frac{(4-x)x' + x(4-x)'}{(4-x)^2}\right) $
Evaluate at $ x = 2 $:
$ g^{-1}(2) = 0 $ means $ \left(g^{-1}(2) + 1\right)^2 + 3 = 4 $.
At $ x = 2 $:
$ \frac{d}{dx} f(g^{-1}(x))\big|_{x=2} = \frac{2}{4} \cdot \frac{2}{4} = \frac{1}{4} = 0.25 $
Therefore, the value of the derivative of the composite function $ f \circ g^{-1} $ at $ x = 2 $ is $ 0.25 $.
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