P(x0,y0)A(x1,y1)B(x2,y2)在抛物线上
y0^2=2px0,y1^2=2px1,y2^2=2px2
y2^2-y1^2=2px2-2px1
(y2-y1)(y2+y1)=2p(x2-x1)
(y2-y1)/(x2-x1)=2p/(y2+y1)
同理：(y2-y0)/(x2-x0)=2p/(y2+y0)
(y1-y0)/(x1-x0)=2p/(y1+y0)
而PA与PB的斜率存在且倾斜角互补
所以,2p/(y2+y0)+2p/(y1+y0)=0
y2+y0=-(y1+y0)
y1+y2=-2y0
AB的斜率=(y2-y1)/(x2-x1)=2p/(y2+y1)=2p/(-2y0)=-p/y0
是非零常数
求（y0+x0）/y0的值----------应该是题目错