.... ...............
cos2х+cosх=0
cos²х-sin²x+cosх=0
2cos²x+cosx-1=0
cosx=(-1±√(1+8))/2=(-1±3)/4
cosx=-1, x=π+2πk; k∈Z
cosx=0.5; x=arccos0.5+2πn n∈Z
х=π/3+2πn; n∈Z
ответ
x=π+2πk; k∈Z
х=π/3+2πn; n∈Z;
