Question
Sin $(\tan^{–1} x), | x | < 1$ is equal to

Answer

Let $\tan^{-1} x = y,$ then $\tan y = x  \Rightarrow \sin y = \frac{x}{\sqrt{1+x^{2}}}$
$\therefore  y = \sin^{-1}  \left(\frac{x}{\sqrt{1+x^{2}}}\right)$
$\Rightarrow \tan ^{-1} x=\sin ^{-1}\left(\frac{x}{\sqrt{1+x^{2}}}\right)$
$\Rightarrow \sin (\tan^{-1} x) = \sin \left(\sin ^{-1}\left(\frac{x}{\sqrt{1+x^{2}}}\right)\right)$
$= \frac{x}{\sqrt{1+x^{2}}}$

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