L與拋物線y^2=8x交于A,B兩點,線段AB的中點M(1,-1)
xA+xB=2xM=2,yA+yB=2yM=-2
k(L)=(yA-yB)/(xA-xB)
y^2=8x
(yA)^2=8xA.(1)
(yB)^2=8xB.(2)
(1)-(2):
(yA+yB)*(yA-yB)=8(xA-xB)
(yA+yB)*(yA-yB)/(xA-xB)=8
-2k(L)=8
k(L)=-4
L:y+1=-4(x-1)
直線l的方程:y=-4x+3