(a)
$\tan^{- 1}{(1) + \tan^{- 1}{(0) + \tan^{- 1}{(2) + \tan^{- 1}{(3)}}}}$
$= \frac{\pi}{4} + \pi + \tan^{- 1}\left( \frac{2 + 3}{1 - 2 \bullet 3} \right)$ (as 2$\bullet$3>1)
$= \frac{5\pi}{4} + \tan^{- 1}{( - 1) = \frac{5\pi}{4} - \frac{\pi}{4} = \pi}$