设椭圆的参数方程为x=acost,y=bsint
令A（acost1,bsint1) B(acost2,bsint2),P(a(cost1+cost2)/2,b(sint1+sint2)/2)
Kab=b(sint1-sint2)/a(cost1-cost2) Kop=b(sint1+sint2)/a(cost1+cost2)
Kab*kOP=b²(sin²t1-sin²t2)/a²(cos²t1-cos²t2)=b²(1-cos²t1-1+cos²t2)/a²(cos²t1-cos²t2)=-b²/a²