设圆心坐标为(x0,y0)则：x0^2+(y0-5)^2=1/5(x0-2y0)^2=1/5(2x0+y0)^25x0^2+5(y0-5)^2=x0^2-4x0y0+4y0^2=4x0^2+4x0y0+y0^2解之得：（1,3）或（5,15）圆的方程为：(x-1)^2+(y-3)^2=5 或 (x-5)^2+(y-15)^2=125...