Question:
The function $f(x)=x^{x}$ has a stationary point at
(a) $x=e$
(b) $x=\frac{1}{e}$
(c) $x=1$
(d) $x=\sqrt{e}$
Solution:
The values of $x$ for which $f^{\prime}(x)=0$ are called stationary values.
Let $f(x)=x^{x}$.
$f(x)=x^{x}$
$\Rightarrow \log f(x)=\log x^{x}$
$\Rightarrow \log f(x)=x \log x$
Differentiating both sides with respect to x, we get
$\frac{1}{f(x)} \times f^{\prime}(x)=x \times \frac{1}{x}+\log x \times 1$
$\Rightarrow f^{\prime}(x)=x^{x}(1+\log x)$
For stationary point, we have
$f^{\prime}(x)=0$
$\Rightarrow x^{x}(1+\log x)=0$
$\Rightarrow 1+\log x=0 \quad\left[x^{x}>0\right]$
$\Rightarrow \log x=-1$
$\Rightarrow x=e^{-1}=\frac{1}{e}$
Thus, the function $f(x)=x^{x}$ has a stationary point at $x=\frac{1}{e}$.
Hence, the correct answer is option (b).