Question

Преобразуй произведение в сумму cos(10x+5y)⋅cos(3x−13y)

Answer 1

$frac{1}{2}(cos(10x+5y+3x-13y)+cos(10x+5y-3x+13y))=frac{1}{2}(cos(13x-8y)+cos(7x+18y))$