P(x,0)
|PA|^2=|PB|^2
(x-2)^2=(x-xB)^2+(yB)^2
x^2-4x+4=x^2-2x*xB+(xB)^2+(yB)^2
4-4x=-2x*xB+1,x≠0
xB=(4x-3)/2x
1≥xB≥-1
点P轨迹:1≥(4x-3)/2x≥-1,x≠0