根据多边形的n个点求出多边形的面积

很方便，两两相邻原点向量求差积，处以二即是面积。

#include<iostream>
#include<cstring>
#include<algorithm>
#include<string>
#include<cmath>
#include<queue>
#include<cstdio>
using namespace std;
const int MAXN=105;
int n;
int main()
{
    cin>>n;
    double x0,y0;
    double x1,y1;
    double x2,y2;
    cin>>x1>>y1;
    x0=x1;
    y0=y1;
    double sum=0;
    for(int i=2;i<=n;i++)
    {
        cin>>x2>>y2;
        sum+=(double)(x1*y2-x2*y1);
        x1=x2;
        y1=y2;
    }
    x2=x0;
    y2=y0;
    sum+=(double)(x1*y2-x2*y1);
    sum/=2;
    cout<<sum<<endl;
    return 0;
}