Помогите~~~ Cos5x-cos4x*cosx=0

Question

Помогите~~~
Cos5x-cos4x*cosx=0

Answer 1

Cos 5x = cos(4x + x) = cos x*cos 4x - sin x*sin 4x
cos x*cos 4x - cos 5x = 0
cos x*cos 4x - cos x*cos 4x + sin x*sin 4x = 0
sin x*sin 4x = 0
sin x = 0, x1 = Pi*k
sin 4x = 0, 4x = Pi*k, x2 = Pi/4*k