y=[(sinx)^2+(cosx)^2]^2-2(sinx)^2(cosx)^2
=1-2*[(sin2x)/2]^2
=1-(sin2x)^2/2
=1-(1/2)*(1-cos4x)/2
=1-1/4+(1/4)cos4x
=(1/4)cos4x+3/4
所以最小正周期T=2π/4=π/2