If $y=\left(\frac{1}{x}\right)^x$, then value of $e^e\left(\frac{d^2 y}{d x^2}\right)_{x=e}$ is:
Answer & explanation
Correct answer: option 2
$y=\left(\frac{1}{x}\right)^x$ ........(1)
taking log on both sides
⇒ $\log y = \log\left(\frac{1}{x}\right)^x$
⇒ $\log y = -x\log x$
differentiating both sides wrt x
so $\frac{1}{x} \frac{dy}{dx} = -\log x - \frac{x}{x}$
so $\frac{dy}{dx} = y(-\log x -1)$ ......(2)
differentiating both sides wrt x
$\frac{d^2 y}{d x^2}=\frac{d y}{d x} (-\log x - 1) + y(-\frac{1}{x})$ .....(3)
$y]_{x=e}=\left(\frac{1}{e}\right)^e=\frac{1}{e^e}$ .....(4)
$\left.\frac{d y}{d x}\right]_{x=e} =\frac{1}{e^e}(-\log e-1)$ ........(5)
$=\frac{-2}{e^e}$
so from (3) at x = e
$\left.\frac{d^2 y}{d x^2}\right]_{x=e} =\frac{-2}{e^e}(-\log e-1)+\frac{1}{e^e}(\frac{-1}{e})$
$=\frac{4}{e^e}+\frac{1}{e^e}\left(\frac{-1}{e}\right)$
so $\left.e^e \frac{d^2 y}{d x^2}\right]_{x=e}=4 -\frac{1}{e}$