(b)
$\because y = \tan^{- 1}\left( \frac{a\cos{x - b\sin x}}{b\cos{x + a\sin x}} \right)$
$= \tan^{- 1}\left( \frac{\frac{a}{b} - \tan x}{1 + \frac{a}{b}\tan x} \right)$
$= \tan^{- 1}\left\lbrack \tan\left( \tan^{- 1}{\left( \frac{a}{b} \right) - x} \right) \right\rbrack$
$\Rightarrow y = \tan^{- 1}{\left( \frac{a}{b} \right) - x}$
$\therefore\frac{dy}{dx} = 0 - 1 = - 1$