(b)
Let $u = \sin^{- 1}{\left( \frac{2x}{1 + x^{2}} \right) = 2\tan^{- 1}x}$
$\therefore\frac{du}{dx} = \frac{2}{1 + x^{2}}$
and $v = \tan^{- 1}{\left( \frac{2x}{1 - x^{2}} \right) = 2\tan^{- 1}x}$
$\therefore\frac{dv}{dx} = \frac{2}{1 + x^{2}}$
Hence, $\frac{du}{dv} = \frac{\left( \frac{du}{dx} \right)}{\left( \frac{dv}{dx} \right)} = 1$