1)x∈(2πn;π+2πn,n∈z)
sinx:sinx/cosx=cos3x
cosx=cos3x
cos3x-cosx=0
-2sinx*sin2x=0
sinx=0⇒x=πn,n∈z
sin2x=0⇒2x=πn,n∈z⇒x=πn/2,n∈z
x=πn,n∈z
2)x∈(π+2πn;2π+2πn,n∈z)
-cosx=cos3x
cos3x+cosx=0
2cosx*cos2x=0
cosx=0⇒x=π/2+πn,n∈z
cos2x=0⇒2x=π/2+πn⇒x=π/4+πn/2,n∈z
x=π/4+2πn,n∈z