设两个端点为（x1,y1）(x2,y2)那半径为1/2√(y2-y1)^2-(x2-x1)^2圆心即线段的中点为((x1+x2)/2，(y1+y2)/2)圆的方程有(x-a)^2+(y-b)^2=r^2代入得(x-(x1+x2)/2)^2+(y-(y1+y2)/2))^2=((y2-y1)^2-(x2-x1)^2)/4
