(b)
$\tan{(\alpha + \beta) = \frac{\tan{\alpha + \tan\beta}}{1 - \tan{\alpha\tan\beta}}}$
$= \frac{\frac{m}{m + 1} + \frac{1}{2m + 1}}{1 - \frac{m}{m + 1}.\frac{1}{2m + 1}}$
$= \frac{2m^{2} + m + m + 1}{2m^{2} + 3m + 1 - m} = 1 = \tan\frac{\pi}{4}$
$\therefore\ \ \ \alpha + \beta = \frac{\pi}{4}$