转载：https://www.cnblogs.com/MaZai/p/10280674.html
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void Calculate_cicular(Point px1, Point px2, Point px3,out Point center,out double r)
{
int x1, y1, x2, y2, x3, y3;
int a, b, c, g, e, f;
x1 = px1.X;//定义点的坐标
y1 = px1.Y;
x2 = px2.X;
y2 = px2.Y;
x3 = px3.X;
y3 = px3.Y;
e = 2 * (x2 - x1);
f = 2 * (y2 - y1);
g = x2 * x2 - x1 * x1 + y2 * y2 - y1 * y1;
a = 2 * (x3 - x2);
b = 2 * (y3 - y2);
c = x3 * x3 - x2 * x2 + y3 * y3 - y2 * y2;
center = new Point();
center.X = (g * b - c * f) / (e * b - a * f);
center.Y = (a * g - c * e) / (a * f - b * e);
r = Math.Sqrt((center.X - x1) * (center.X - x1) + (center.Y - y1) * (center.Y - y1));
}