|P1P2|^2=(2+sinθ-cosθ)^2+(2-cosθ-sinθ)^2
=4+4(sinθ-cosθ)+(sinθ-cosθ)^2+4-4(sinθ+cosθ)+(sinθ+cosθ)^2
=8-8cosθ+2
=10-8cosθ
最大为sqrt18
|P1P2|^2=(2+sinθ-cosθ)^2+(2-cosθ-sinθ)^2
=4+4(sinθ-cosθ)+(sinθ-cosθ)^2+4-4(sinθ+cosθ)+(sinθ+cosθ)^2
=8-8cosθ+2
=10-8cosθ
最大为sqrt18