cos7x-cos3x= -(sin8x+sin2x)
-2sin5x sin2x= -2sin5x cos3x
2sin5x (cos3x-sin2x)=0
1) sin5x=0, 5x=πn, x=πn/5, n∈Z
2)cos3x-sin2x=0, cos3x-cos(π/2-2x)=0, -2sin(π/2-x) sin(5x-π/2)=0
sin(π/2-x)=0, cosx=0, x=π/2+2πk, k∈Z
sin(5x-π/2)=0, -sin(π/2-5x)=0, cos5x=0, 5x=π/2+2πm, x=π/10+2πm/5, m∈Z