Question

Вычилите sina*sin3a, если cos2a=-1/3

Answer 1

Sin(4a + 2a)/2 Sin(4a - 2a)/2 = -1/2(Сos 4a - Cos 2a)= 
=-1/2(2Cos² 2a -1 - Cos 2a) = -1/2·(2·1/9 -1 +1/3)=-1/2·(-4/9) = 2/9