圆A: (x+2)^2+y^2=1
A(-2,0)
P(x,y)
|PA|-1=1-x
√[(x+2)^2+y^2] -1=(1-x)
(x+2)^2+y^2=(2-x)^2
动圆圆心P轨迹:y^2=4x