1+cos7x=(sin1,5x-cos1,5x)^2 help me ?

1+cos7x=(sin1,5x-cos1,5x)^2 
1+cos7x=sin²(1.5x)-2sin(1.5x)cos(1.5x)+cos²(1.5x)
cos7x=-sin3x
cos7x=cos(3x+π/2)
3x+π/2=±arccos(cos7x)+2πn
3x+π/2=±7x+2πn
x(3±7)=-π/2+2πn
x=(-π/2+2πn)/(3±7), n∈Z
ответ:
x=(-π/2+2πn)/10
x=-(-π/2+2πn)/4
n∈Z