помогите решить сosx+cos3x=0

cos(3x) = cos(x)*cos(2x) - sin(x)sin(2x)
==>
cos(x) (1+cos(2x)) - sin(x)*sin(2x) = 0
2cos(x)*cos^2(x) - sin(x)*2sin(x)*cos(x) = 0
cos(x) ( cos^2(x) - sin^2(x)) = 0
cos(x) * cos(2x) = 0
==> x= pi/2 +pi*n
x= pi/4 +pi/2 * k