设点M(y²1/4,y1).P(y²2/4,y2) A、M、P三点共线
Kam=Kmp
y1/(y²1/4+1)=(y1-y2)/(y²1/4-y²2/4)化简整理得y1y2=4 (1)
设点Q(y²3/4,y3) M.B.Q三点共线
Kbq=Kmq
(y3+1)/(y²3/4-1)=(y3-y1)/(y²3/4-y²1/4)化简整理得y1y3+y1+y3+4=0 (2)
由（1）得y1=4/y2代入（2）得4y3/y2+4/y2+y3+4=0
整理得 4(y2+y3)+y2y3+4=0 (3)
Kpq=(y2-y3)/(y²2/4-y²3/4)=4/(y2+y3)
直线PQ的方程是y-y2=4/(y2+y3)(x-y²2/4)
(y-y2)(y2+y3)=4x-y²2
y(y2+y3)-y2y3=4x
将（3）代入整理得
y(y2+y3)+4(y2+y3)=4x-4
(y+4)(y2+y3)=4(x-1)
(y+4)=[4/(y2+y3)](x-1)
直线PQ恒过一个定点(1,-4)