y=sinxcosx+cos²x-1/2
=(1/2)·2sinxcosx+(1/2)(2cos²x-1)
=(1/2)sin2x+(1/2)cos2x
=(√2/2)[sin2x·cos(π/4)+cos2x·sin(π/4)]
=(√2/2)sin(2x+π/4)
最小正周期T=2π/2=π
值域[-√2/2,√2/2]