平面坐标系下计算3个点的角度

public  static  float  Angle(Point cen, Point first, Point second)

        {
            float  dx1, dx2, dy1, dy2;
            float  angle;

            dx1 = first.X - cen.X;
            dy1 = first.Y - cen.Y;

            dx2 = second.X - cen.X;

            dy2 = second.Y - cen.Y;

            float  c = ( float )Math.Sqrt(dx1 * dx1 + dy1 * dy1) * ( float )Math.Sqrt(dx2 * dx2 + dy2 * dy2);

            if  (c == 0)  return  -1;

            angle = ( float )Math.Acos((dx1 * dx2 + dy1 * dy2) / c);




            return  angle;
        }