HDU ACM 11 2036 改革春风吹满地

向量叉求面积。

设三点A，B，C。

S = (AxBy + BxCy + CxAy - Axcy - Bxay - CxBy)/2;

#include <iostream>using namespace std;struct Lu{double x;double y;};int main(){Lu Hui[100];int n;while (cin >> n && n){for (int i = 0; i < n; i++)cin >> Hui[i].x >> Hui[i].y;double Yi = 0;for (int i = 1; i < n - 1; i++)Yi += (Hui[0].x * Hui[i].y + Hui[i].x * Hui[i + 1].y + Hui[i + 1].x * Hui[0].y - Hui[0].x * Hui[i + 1].y - Hui[i].x * Hui[0].y - Hui[i + 1].x * Hui[i].y)/2.0;cout << fixed;cout.precision(1);cout << Yi << endl;}return 0;}