cosx cos4x cos5x=1 алгебра решить

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