1-cos2x
=1-(1-2sin²x)
=2sin²x
1+cos2x
=1+2cos²x-1
=2cos²x
所以f(x)=√2(|sinx|+|cosx|)
f(x+π/2)=√2(|cosx|+|-sinx|)=f(x)
T=π/2