Question

Cos7a - cos5a на множники

Answer 1

$\displaystyle cos(7a)-cos(5a)=-2sin(\frac{7a+5a}{2})sin(\frac{7a-5a}{2})=-2sin(\frac{12a}{2})sin(\frac{2a}{2})=\\ \\ -2sin(6a)sin(a)$