Cos(a)+cos(b) = 2·cos(½(a+b))·cos(½(a-b))
cos2x + cos3x = 2*cos 5x/2 * cos ( -x/2)
2*cos 5x/2 * cos ( -x/2) = 0
cos 2,5x * cos ( -x/2) = 0
1) cos2,5x = 0
2,5x = pi/2 + pik
x = pi/5 + (2pik)/5, k ∈ Z
2) cos( - x/2) = 0
cos (x/2)= 0
x/2 = pi/2 + pik
x = pi + 2pik, k ∈Z