准线：x=-1
设A(x1,y1),B(x2,y2)
A,B 到焦点距离之和=A,B 到准线距离之和
=(x1+1)+(x2+1)
=(x1+x2)+2
=5
x1+x2=3
(x1+x2)/2=3/2
线段AB的中点的横坐标:3/2