(a)
We have, $\tan{(\alpha + \beta) = \frac{\tan{\alpha + \tan\beta}}{1 - \tan{\alpha\tan\beta}}}$
$= \frac{\frac{1}{\sqrt{x(x^{2} + x + 1)}} + \frac{\sqrt{x}}{\sqrt{x^{2} + x + 1}}}{1 - \frac{1}{\sqrt{x(x^{2} + x + 1)}}.\frac{\sqrt{x}}{\sqrt{x^{2} + x + 1}}}$
$= \frac{(1 + x)\sqrt{x^{2} + x + 1}}{\sqrt{x}.x(x + 1)}$
$= \sqrt{x^{- 3} + x^{- 2} + x^{- 1}} = \tan{\gamma\ (given)}$
$\therefore\ \alpha + \beta = \gamma$