f(x)=0.5cos平方x-sinxcosx-0.5sin平方x =0.5((cosx)^2-(sinx)^2)-0.5*2sinxcosx=0.5cos2x-0.5sin2x=0.5(cos2x+cos(2x-π/2))=cos(2x-π/4)sin(π/4)=√2/2cos(2x-π/4) 最小正周期为π