f(x)=cosx+cosxcosπ/3-sinxsinπ/3
=cosx+cosx*1/2-sinx*√3/2
=cosx*3/2-sinx*√3/2
=√3(cosx*√3/2-sinx*1/2)
=√3(cosxcosπ/6-sinxsinπ/6)
=√3cos(x+π/6)