MCQ
If $\text{f}(\text{x})=\tan^{-1}(\text{g}(\text{x})),$ where g(x) is monotonically increasing for $0<\text{x}<\frac{\pi}{2}.$ Then, f(x) is:
  • Increasing on $\Big(0,\frac{\pi}{2}\Big)$
  • B
    Decreasing on $\Big(0,\frac{\pi}{2}\Big)$
  • C
    Increasing on $\Big(0,\frac{\pi}{4}\Big)$ and decreasing on $\Big(\frac{\pi}{4},\frac{\pi}{2}\Big)$
  • D
    None of these.

Answer

Correct option: A.
Increasing on $\Big(0,\frac{\pi}{2}\Big)$
Given: g(x) is increasing on $\Big(0,\frac{\pi}{2}\Big).$ Then,
$\text{x}_1<\text{x}_2,\forall\ \text{x}_1<\text{x}_2\in\Big(0,\frac{\pi}{2}\Big)$
$\Rightarrow\text{g}(\text{x}_1)<\text{g}(\text{x}_2)$
Taking $\tan^{-1}$ on both sides, we get
$\Rightarrow\tan^{-1}(\text{g}(\text{x}_1))<\tan^{-1}(\text{g}(\text{x}_2))$
$\Rightarrow\text{f}(\text{x}_1)<\text{f}(\text{x}_2),\ \forall\ \text{x}_1,\text{x}_2\in\Big(0,\frac{\pi}{2}\Big)$
So, f(x) is increasing on $\Big(0,\frac{\pi}{2}\Big).$

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