cos2x=2cosx^2-1
cosx^2=(cos2x+1)/2
f(x)=√3／2sin2x-cos^2x-1／2 =√3／2sin2x-1/2cos^2x-1=sin(2x-pai/6)-1
所以最小值=-1-1=-2
最小正周期t=2π/2=π