AB是抛物线Y方＝4X的焦点弦
设为：y=k(x-1)
则：y1+y2=k(x1+x2)-2k=6k-2k=4k
y1^2=4x1,y2^2=4x2
(y1^2-y2^2)=4(x1-x2)
(y1-y2)/(x1-x2)=4/(y1+y2)
即：k=4/4k
k^2=1
k=±1
直线AB的斜率是:±1