三点计算圆心和半径

            double[] x = { 338.5,397.5,275.5};
            double[] y = { 199.5, 286.5, 310.5 };
            double u = (y[1] - y[0]) / (x[1] - x[0]);
            double v = (y[2] - y[1]) / (x[2] - x[1]);
            double w = 0.5 * (y[0] + y[1]) + 0.5 * (x[0] + x[1]) / ((y[1] - y[0]) / (x[1] - x[0]));
            double k = 0.5 * (y[1] + y[2]) + 0.5 * (x[1] + x[2]) / ((y[2] - y[1]) / (x[2] - x[1]));

            double cx = (k - w) / (1 / v - 1 / u);
            double cy = (w - k) / u / (1 / v - 1 / u) + w;
            double r = Math.Sqrt(Math.Pow((y[0] - cy), 2) + Math.Pow((x[0] - cx), 2));
            Console.WriteLine("x  {0},   y  {1},  r   {2}",cx,cy,r);
            Console.ReadKey();