#include <iostream>#include <cmath>using namespace std;struct Point{    float x;     float y;    Point(float a, float b) : x(a), y(b)    {    }};double Length(Point & A, Point & B){    return sqrt(pow(A.x - B.x, 2) + pow(A.y - B.y, 2));}double Area1(Point & A, Point & B, Point & C){    double a, b, c;    a = Length(A, B);    b = Length(B, C);    c = Length(C, A);    double p = (a + b + c) / 2;    return sqrt((p - a) * (p - b) * (p - c) * p);//海伦公式计算三角形面积}double CrossProduct(Point & v1, Point & v2){    return v1.x * v2.y - v1.y * v2.x;}int main(){    Point A(10, 2);    Point B(3, 4);    Point C(5, 7);    Point v1(B.x - A.x, B.y - A.y);  //向量AB    Point v2(C.x - A.x, C.y - A.y);  //向量AC    cout << CrossProduct(v1, v2) << endl;    cout << Area1(A, B, C) << endl;    return 0;}