设P(x,y)
PA=√[(x+2)^2+y^2]
PB=√[(x-1)^2+y^2]
PA=√2PB
√[(x+2)^2+y^2]=√2√[(x-1)^2+y^2]
x^2-6x-2+y^2=0
x^2-6x+9+y^2=11
(x-3)^2+y^2=11
P轨迹是圆
圆心是(3,0)半径是√11
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