y=(cosx)^2-2√3sinxcosx=(cox2x+1)/2-√3sin2x
=cos2x/2-√3sin2x+1/2
=-√13/2*sin(2x-φ)+1/2
其中tanφ=√3/6,
∴y的值域为[(1-√13)/2,(1+√13)/2]