f(x)=cos(2x-π/3)-cos2x-1
= cos2xcosπ/3+sin2xsinπ/3-cos2x-1
= 1/2cos2x+√3/2sin2x-cos2x-1
= -1/2cos2x+√3/2sin2x-1
= -cos2xcosπ/3+sin2xsinπ/3-1
= -cos(2x+π/3)-1