y＝cos^2(x＋π4)－sin^2(x＋π/4)
= cos{2(x+π/4)}
=cos(2x+π/2)
最小正周期=2π/2=π最小正周期=2π/2=π 何解？不明白公式啊！y=Asin(wx+θ)的最小正周期=2π/w