Sin (a+60) sin (a-60) - sin^2a

помогите решить

Вспоминаем формулу sin (a + b) = sin (a) cos (b) - sin (b) cos (a)

sin (a + 60°) sin (a - 60°) = (sin a cos 60° + cos a sin 60°) (sin a cos 60° - cos a sin 60°) = sin^2 a cos^2 60° - cos^2 a sin^2 60° = sin^2 a * 1/4 - cos^2 a * 3/4 = sin^2 a * 1/4 - (1 - sin^2 a) * 3/4 = sin^2 a - 3/4

sin^2 a - 3/4 - sin^2 a = - 3/4