y^2=2px
设直线 x=ty+m
联立得 y^2=2p(ty+m)
y^2-2pty-2pm=0
y1y2=-2pm
(y1y2)^2 =4p^2m^2=4p^2 (x1)(x2) x1x2=m^2
y1y2/x2x2= -2pm/m^2 =-2p/m
y^2=2px
设直线 x=ty+m
联立得 y^2=2p(ty+m)
y^2-2pty-2pm=0
y1y2=-2pm
(y1y2)^2 =4p^2m^2=4p^2 (x1)(x2) x1x2=m^2
y1y2/x2x2= -2pm/m^2 =-2p/m