The minimum value of \(\displaystyle\frac{x}{\log x}\ (x \gt 1)\) is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → e
Let \(f(x) = \frac{x}{\log x}\). By the quotient rule:
\( \displaystyle f'(x) = \frac{\log x - x \cdot \frac{1}{x}}{(\log x)^2} = \frac{\log x - 1}{(\log x)^2} \)
\(f'(x) = 0 \Rightarrow \log x = 1 \Rightarrow x = e\)
For 1 < x < e, log x < 1, so f'(x) < 0; for x > e, f'(x) > 0. By the first derivative test, f has its minimum at x = e:
\( \displaystyle f(e) = \frac{e}{\log e} = \frac{e}{1} = e \)