一个半小时没调试出错误在哪！！！！！！！！！！！！！！！！

/// <summary>
        /// 图像区画弧
        /// </summary>
        /// <param name="gs"></param>
        /// <param name="startpoint"></param>
        /// <param name="endpoint"></param>
        /// <param name="point"></param>
        public static void DrawArc(Graphics gs,PointF startpoint,PointF endpoint,PointF pointt)
        {
            PointF point = MyArc.ArcCircle(startpoint, endpoint, pointt);//求圆心坐标，测试无错误
            float a = startpoint.X-point.X;
            float b = startpoint.Y-point.Y;
            float c = endpoint.X-point.X;
            float d = endpoint.Y-point.Y;//以上将圆心作为原点，以方便计算
            double startangle;
            double endangle;
            if (b != 0&&d!=0&&a!=0&&c!=0)
            {
                float kstart = -a / b;
                float kend = -c / d;
                double startradian= Math.Atan((double)kstart);
                double endradian= Math.Atan((double)kend);
                if (a < 0)//起始点与原点连线与X轴成钝角
                {
                     startangle = startradian * 180 / Math.PI + 180;
                }
                else//起始点与原点连线与X轴成锐角
                {
                    startangle = startradian * 180 / Math.PI;
                }
                if (c < 0)//终点与原点连线与X轴成钝角
                {
                    endangle = endradian * 180 / Math.PI + 180;
                }
                else//终点与原点连线与X轴成锐角
                {
                    endangle = endradian * 180 / Math.PI;
                }
               gs.DrawArc(Pens.Red, point.X - r, point.Y - r, r * 2, r * 2, (float)startangle, (float)endangle);
            }        }//以上是代码的部分，错误出在这里，谁帮忙看看啊，谢拉