$\mathrm{F}(\mathrm{x})=\log |\sin \mathrm{x}|$, where $\mathrm{x} \in(0, \pi)$ is strictly increasing on

$\mathrm{F}(\mathrm{x})=\log |\sin \mathrm{x}|$, where $\mathrm{x} \in(0, \pi)$ is strictly increasing on
  1. $\left(\frac{\pi}{2}, \pi\right)$ only
  2. $(0, \pi)$ only
  3. $\left(0, \frac{\pi}{2}\right)$ only
  4. $\left(\frac{\pi}{4}, \frac{3 \pi}{4}\right)$ only

Solution

$f(x)=\log |\sin x|$, where $x \in(0, \pi)$ $\therefore \mathrm{f}^{\prime}(\mathrm{x})=\frac{1}{\sin \mathrm{x}} \times \cos \mathrm{x}=\cot \mathrm{x}$ When $\mathrm{f}^{\prime}(\mathrm{x})>0$, we say $\frac{\cos \mathrm{x}}{\sin \mathrm{x}}>0$ Here $\sin x>0 \ldots[x \in(0, \pi)]$ for the function to be strictly increasing, $\cos x>0$ $\rightarrow \mathrm{x} \in\left(0, \frac{\pi}{2}\right)$ only.

Asked in: MHT CET 2021 (23 Sep Shift 1)

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