(b)
$\because\ \ \ \ \ y = \sec^{- 1}{\left( \frac{x + 1}{x - 1} \right) + \sin^{- 1}\left( \frac{x - 1}{x + 1} \right)}$
$= \cos^{- 1}{\left( \frac{x - 1}{x + 1} \right) + \sin^{- 1}\left( \frac{x - 1}{x + 1} \right)\ }$
$\Rightarrow$ $y$ = $\frac{\pi}{2}\ \ \ \ \ \Rightarrow \ \ \frac{dy}{dx}$ =0