向量P1P2=向量OP2-向量OP1
=（1+sinθ,1-cosθ)-(cosθ,sinθ)
=(1+sinθ-cosθ,1-cosθ-sinθ)
|向量P1P2|
=√[(1+sinθ-cosθ)²+(1-cosθ-sinθ)²]
=√[(1+sinθ-cosθ)²+(1-cosθ-sinθ)²]
=√[2+2(sin²θ+cos²θ)-4cosθ]
=√(4-4cosθ)≤2√2
B.2倍根号2