求多边形面积公式（已经知道顶点坐标）

链接

int t;
	cin>>t;
    
    
    
    
	while(t--) {
		int n;
		cin>>n;
		for(int i=1;i<=n;i++){
			double x,y,x0,y0,sum = 0.0;
			cin >> x >> y;
			x0 = x;
			y0 = y;
			while (--n) {
				double xtmp,ytmp;
				cin >> xtmp >> ytmp;
				sum += (x*ytmp-y*xtmp);
				x = xtmp;
				y = ytmp;
			}
			sum += (x*y0-y*x0);
			printf("%.lf\n",floor(fabs(sum/2)));
		}
	}