Use de Moivre's formula (Sec. 7) to derive the following trigonometric identities: (a) cos 3 = cos3

Use de Moivre's formula (Sec. 7) to derive the following trigonometric identities:
(a) cos 3θ = cos3 θ − 3 cos θ sin2 θ;
(b) sin 3θ = 3 cos2 θ sin θ − sin3 θ.