设圆心O坐标为（X,Y）,圆心到ABC三点距离相等为圆的半径R
则OA=OB＝OC＝R
OA^2＝OB^2＝OC^2
(X+2)^2+(Y-4)^2=(X+1)^2+(Y-3)^2=(X-2)^2+(Y-6)^2=R^2
求出X-Y=-5
2X+Y=5
X=0 Y=5
R^2=4+1=5 R=√5
所以圆为X^2+（Y-5）^2=5