Question

Помогите решить! cos x * cos 2x = cos 3x

Answer 1

$cos x cos 2x = cos 3x \ frac{1}{2} (cos3x+cosx)=cos3x \ cos3x+cosx=2cos3x \ cos3x-cosx=0 \ -2sin2xsinx=0 \ sin2x=0 \ x_1= frac{pi n}{2}, n in Z \ sinx=0 \ x_2=pi n, n in Z$