Cosx + Cosy = 2Cos((x+y)/2)Cos((x-y)/2)
Cosx - Cosy = -2Sin((x+y)/2)Sin((x-y)/2)
2Cos3x*Cos2x = 2Cos5x*Cos2x
2Cos2x(Cos5x - Cos3x)=0
-4Cos2x*Sin4x*Sinx = 0
Cos2x =0,⇒ 2x = π/2 + πk,⇒ x= π/4 + πk/2 k∈Z
Sin4x = 0,⇒ 4x = πk, x = πk/4
Sinx = 0,⇒ x = πk
x= πk/4, x = πk, k∈Z