![在这里插入图片描述](https://img-blog.csdnimg.cn/626c7074626443e28509e00bfa414b17.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBAc3NsbHly,size_20,color_FFFFFF,t_70,g_se,x_16)

分析

叉积求面积，乘上B。每个三角形面积相加，最后取绝对值

上代码

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<iomanip>
using namespace std;double n,b,x[100011],y[100011],ans;double cj(double x1,double y1,double x2,double y2)
{return x1*y2-x2*y1;
}int main()
{cin>>n>>b;for(int i=1;i<=n;i++){cin>>x[i]>>y[i];}for(int i=1;i<=n;i++){int j=i+1;if(j==n+1) j=1;ans+=0.5*cj(x[i],y[i],x[j],y[j]);}printf("%.4lf",abs(ans*b));return 0;}