Cos(1,5пи+2x) - cosx=0

sin2x-sin(π/2-x)=0
2sin(3x/2-π/4)cos(x/2+π/4)=0
sin(3x/2-π/4)=0
3x/2-π/4=πn
3x/2=π/4+πn
x=π/6+2πn/3
cos(x/2+π/4)=0
x/2+π/4=π/2+πn
x/2=π/4+πn
x=π/2+2πn