解由题知定圆b:(x-4)^2+ y^2=16的圆心为B(4,0)半径为4设所求的动圆的圆心为P(x，y)半径为R则PB=PA+4(外切)或PA=PB+4(内切)则√[(x-4)^2+ y^2]=√[(x+4)^2+ y^2]+4或√[(x+4)^2+ y^2]=√[(x-4)^2+ y^2]+4下面换件整理即可
