设P点坐标为（x,y)
则PA=√(x+3)^2+y^2
PB=√(x-3)^2+y^2
则
PA:PB=√(x+3)^2+y^2:√(x-3)^2+y^2=1:2
2√(x+3)^2+y^2=√(x-3)^2+y^2
4(x+3)^2+4y^2=(x-3)^2+y^2
4x^2+24x+36+4y^2=x^2-6x+9+y^2
3x^2+30x+3y^2+27=0
x^2+10x+y^2+9=0
x^2+10x+25+y^2=25-9=16
(x+5)^2+y^2=16