TRIGONOMETRICAL IDENTITIES - PART – 87

121. (i) If 2 sin A – 1 = 0, 

show that sin 3A = 3 sin A – 4 sin³ A.

(ii) If 4 cos² A – 3 = 0, 

show that cos 3A = 4 cos³ A – 3 cos A.