Question

Помогите доказать тождество.

Answer 1

Ответ:

$sin^4(x/2) - cos^4(x/2) = (sin^2(x/2) + cos^2(x/2))(sin^2(x/2) - cos^2(x/2)) = -(cos^2(x/2) - sin^2(x/2)) = -cos(2*(x/2)) = -cos(x)$, ч.т.д.