已知圆上三点坐标，要根据这三点坐标画圆弧，找了半天，也没发现完善的现成代码，想了半天算法，发现自己已经把解析几何忘了个差不多了。没办法最后根据余弦定理自己写了一个。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace DrawArc
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private static void CalcDegree(ref Point O, ref Point A, ref Point B, out double angleAB, out double angleA, out double angleB)
        {
            //x轴上的向量
            int Xx = 100;
            int Xy = 0;

            //oa向量
            int ax = A.X - O.X;
            int ay = A.Y - O.Y;

            //ob向量
            int bx = B.X - O.X;
            int by = B.Y - O.Y;

            CalcAngle(ax, ay, Xx, Xy,out angleA);
            CalcAngle(bx, by, Xx, Xy, out angleB);
            CalcAngle(ax, ay, bx, by, out angleAB);

            //坐标变换
            ConvertAngelToOrig(ref O, ref A, ref angleA);
            ConvertAngelToOrig(ref O, ref B, ref angleB);
        }

        private static void CalcAngle(int vx, int vy, int vx2, int vy2, out double angle)
        {
            //double dA = Math.Sqrt(vx * vx + vy * vy);
            //double dB = Math.Sqrt(vx2 * vx2 + vy2 * vy2);
            //double dC = Math.Sqrt((vx2 - vx) * (vx2 - vx) + (vy2 - vy) * (vy2 - vy));

           // angle = Math.Acos((dA * dA + dB * dB - dC * dC) / 2 * dA * dA) * (180 / Math.PI);
            int Point_Mul_a = (vx * vx2) + (vy * vy2);
            double Mul_a = Math.Sqrt(vx * vx + vy * vy) * Math.Sqrt(vx2 * vx2 + vy2 * vy2);

            //计算夹角余弦值
            double Cos_a = Point_Mul_a / Mul_a;
            double A_Cos = Math.Acos(Cos_a);
            //求出几何坐标系中的角度，即按逆时针的方法
            angle = A_Cos * (180 / Math.PI);
        }

        private static void ConvertAngelToOrig(ref Point O, ref Point A, ref double angleA)
        {
            if (A.Y <= O.Y && A.X >= O.X)//1
            {
                angleA = 360 - angleA;
            }
            else if (A.Y <= O.Y && A.X < O.X)//2
            {
                angleA = 360 - angleA;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            PaintArc();
        }

        private void PaintArc()
        {
            //三个点，O为原点，A、B为圆上另外两点
            Point Point_O = new Point(200, 200);
            //Point Point_A = new Point(100, 100);
            //Point Point_B = new Point(300, 100);
            //Point Point_A = new Point(150, 150);
            //Point Point_B = new Point(250, 150);

            //Point Point_A = new Point(250, 150);
            //Point Point_B = new Point(250, 250);

            //Point Point_A = new Point(250, 250);
            //Point Point_B = new Point(150, 150);

            //Point Point_A = new Point(250, 250);
            //Point Point_B = new Point(250, 150);

            Point Point_A = new Point(150, 250);
            Point Point_B = new Point(250, 150);

            double angleA;
            double angleB;
            double angleAB;

            CalcDegree(ref Point_O, ref Point_A, ref Point_B, out angleAB, out angleA, out angleB);

            double distance = Math.Sqrt((Point_A.X - Point_O.X) * (Point_A.X - Point_O.X) + (Point_A.Y - Point_O.Y) * (Point_A.Y - Point_O.Y));

            float topLeftX = (float)(Point_O.X - distance);
            float topLeftY = (float)(Point_O.Y - distance);

            //初始化画板
            Graphics gr = this.CreateGraphics();
            Brush br = new SolidBrush(Color.Red);
            Pen pe = new Pen(Color.Black, 10);

            //画出圆弧

            double delta = angleB - angleA;
            if ((delta > 0 && delta < 180) || (delta < -180))//A->B
            {
                gr.DrawArc(pe, topLeftX, topLeftY, (float)(2 * distance), (float)(2 * distance), (float)(angleA), (float)(angleAB));
            }
            else//B->A
            {
                gr.DrawArc(pe, topLeftX, topLeftY, (float)(2 * distance), (float)(2 * distance), (float)(angleB), (float)(angleAB));
            }
            //画出点
            gr.FillEllipse(br, Point_O.X, Point_O.Y, 7, 7);
            gr.FillEllipse(br, Point_A.X, Point_A.Y, 7, 7);
            gr.FillEllipse(br, Point_B.X, Point_B.Y, 7, 7);

            pe.Dispose();
            br.Dispose();
            gr.Dispose();
        }
    }
}