(b)
$\frac{\sin{x + \sin y}}{\cos{x + \cos y}} = \frac{a}{b}$
$\Rightarrow \ \ \ \frac{2\sin{\left( \frac{x + y}{2} \right)\cos\left( \frac{x - y}{2} \right)}}{2\cos{\left( \frac{x + y}{2} \right)\cos\left( \frac{x - y}{2} \right)}} = \frac{a}{b}$
$\Rightarrow \ \tan\left( \frac{x + y}{2} \right) = \frac{a}{b}$