Explanation

(a)
Using the relation
$\cos{A + \cos{B - \cos{C = - 1 + 4\cos{\frac{A}{2}\cos{\frac{B}{2}\sin\frac{C}{2}}}}}}$
Put $A = 56{^\circ},\ \ B = 58{^\circ},\ \ C = 66{^\circ}$
$\therefore\ \ \cos{56{^\circ} + \cos{58{^\circ} - \cos{66{^\circ}}}}$
$= - 1 + 4\cos{28{^\circ}\cos{29{^\circ}\sin{33{^\circ}}}}$
$\Rightarrow \ \ 1 + \cos{56{^\circ} + \cos{58{^\circ} - \cos{66{^\circ}}}}$
$= 4\cos{28{^\circ}\cos{29{^\circ}\sin{33{^\circ}}}}$