y=(cosxcos60-sinxsin60)(cosxcos120-sinxsin120)
=(1/2cosx-√3/2sinx)(-1/2cosx-√3/2sinx)
=-(1/2cosx-√3/2sinx)(1/2cosx+√3/2sinx)
=-1/4cos^2x+3/4sin^2x
=sin^2x-1/4(sin^2x+cos^2x)
=sin^2x-1/4
当sinx=1时,y取得最大值ymax=1^2-1/4=3/4