For $x \in\left(0, \frac{1}{4}\right)$, if the derivative of $\tan ^{-1}\left(\frac{6 x \sqrt{x}}{1-9…

For $x \in\left(0, \frac{1}{4}\right)$, if the derivative of $\tan ^{-1}\left(\frac{6 x \sqrt{x}}{1-9 x^3}\right)$ is $\sqrt{x} \cdot g(x)$, then $g(x)$ equals
  1. $\frac{9}{1+9 x^3}$
  2. $\frac{3 x}{1-9 x^3}$
  3. $\frac{3 x \sqrt{x}}{1-9 x^3}$
  4. $\frac{3}{1+9 x^3}$

Solution

$\begin{aligned} & y=\tan ^{-1}\left(\frac{6 x \sqrt{x}}{1-9 x^3}\right)=\tan ^{-1}\left\{\frac{2\left(3 x^{\frac{3}{2}}\right)}{1-\left(3 x^{\frac{3}{2}}\right)^2}\right\}=2 \tan ^{-1}\left(3 x^{\frac{3}{2}}\right) \\ & \Rightarrow \frac{\mathrm{d} y}{\mathrm{~d} x}=2 \times \frac{1}{1+9 x^3} \times 3 \times \frac{3}{2} \sqrt{x}=\frac{9}{1+9 x^3} \cdot \sqrt{x} \\ & \Rightarrow g(x)=\frac{9}{1+9 x^3}\end{aligned}$

Asked in: MHT CET 2022 (10 Aug Shift 2)

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