class CalcArea
    {
        //点
        public class Point
        {
            private double x;
            private double y;

            public Point(double a, double b)
            {
                x = a;
                y = b;
            }
            public double getX()
            {
                return x;
            }
            public double getY()
            {
                return y;
            }

            public void setX(double a)
            {
                x = a;
            }
            public void setY(double b)
            {
                y = b;
            }
        }

        //边
        public class Line
        {
            private Point p1;
            private Point p2;

            public Line(Point x, Point y)
            {
                p1 = x;
                p2 = y;
            }
            public Point getP1()
            {
                return p1;
            }
            public Point getP2()
            {
                return p2;
            }

            public void setP1(Point x)
            {
                p1 = x;
            }
            public void setP2(Point x)
            {
                p2 = x;
            }
        }

        //三角形
        public class Triangle
        {
            private Point p1;
            private Point p2;
            private Point p3;

            public Triangle(Point a, Point b, Point c)
            {
                p1 = a;
                p2 = b;
                p3 = c;
            }

            public Point getP1()
            {
                return p1;
            }
            public Point getP2()
            {
                return p2;
            }
            public Point getP3()
            {
                return p3;
            }
            public void setP1(Point x)
            {
                p1 = x;
            }
            public void setP2(Point x)
            {
                p2 = x;
            }
            public void setP3(Point x)
            {
                p3 = x;
            }
        }

        //计算三角形面积
        public class Util
        {
            //计算两点间距离
            public static double dispp(Point p1, Point p2)
            {
                double distX = p1.getX() - p2.getX();
                double distY = p1.getY() - p2.getY();
                return Math.Sqrt(distX * distX + distY * distY);

            }

            //点到直线的距离
            public static double displ(Point p, Line l)
            {

                double lx1 = l.getP1().getX();
                double lx2 = l.getP2().getX();
                double ly1 = l.getP1().getY();
                double ly2 = l.getP2().getY();
                double k = (ly2 - ly1) / (lx2 - lx1);
                double b = ly1 - k * lx1;
                return Math.Abs((p.getY() - k * p.getX() - b)) / Math.Sqrt((k * k + 1));

            }

            //计算面积
            public static double triArea(Triangle t)
            {

                Line l = new Line(t.getP1(), t.getP2());
                return Util.dispp(t.getP1(), t.getP2()) * Util.displ(t.getP3(), l) / 2;

            }


            //向量方法计算
            public static double triArea1(Triangle t)
            {
                double area = 0;
                double a = Util.dispp(t.getP1(), t.getP2());
                double b = Util.dispp(t.getP2(), t.getP3());
                double c = Util.dispp(t.getP1(), t.getP3());
                double d = (a + b + c) / 2;
                area = Math.Sqrt(d * (d - a) * (d - b) * (d - c));
                return area;
            }

        }
    }

        #region testTriangle
        /// <summary>
        /// 计算点在不在三角形内部
        /// </summary>
        private void testTriangle()
        {
            CalcArea.Point p1 = new CalcArea.Point(-1, 1);
            CalcArea.Point p2 = new CalcArea.Point(1, 0.5);
            CalcArea.Point p3 = new CalcArea.Point(2, -3);


            double x = 0.8;

            double y = 0.3;

            CalcArea.Point p4 = new CalcArea.Point(x, y);


            CalcArea.Triangle t;

            t = new CalcArea.Triangle(p1, p2, p3);
            double sourceArea = Convert.ToDouble(CalcArea.Util.triArea(t));

            t = new CalcArea.Triangle(p1, p2, p4);
            double area1 = Convert.ToDouble(CalcArea.Util.triArea1(t));

            t = new CalcArea.Triangle(p3, p2, p4);
            double area2 = Convert.ToDouble(CalcArea.Util.triArea1(t));

            t = new CalcArea.Triangle(p1, p3, p4);
            double area3 = Convert.ToDouble(CalcArea.Util.triArea1(t));

            if (sourceArea == (double)((int)((area1 + area2 + area3) * 100)) / 100)
            {
                Console.WriteLine("点在三角形内部！");
            }
            else
            {
                Console.WriteLine("点不在三角形内部！");
            }

            Console.ReadLine();
        } 
        #endregion