hdu 1115 Lifting the Stone

//31MS    240K    799 B    C++    
//数学，平面薄板面积公式 
#include<stdio.h>
struct node{
    double x,y;
};
double Area(node a,node b,node c) //三点面积公式 
{
    return ((b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y))/2;
}
int main() 
{
    int t,n;
    double a,b;
    scanf("%d",&t);
    while(t--)
    {
        double sum_area=0;
        double sum_x=0;
        double sum_y=0;
        node p0,p1,p2;
        scanf("%d",&n);
        scanf("%lf%lf",&p0.x,&p0.y);
        scanf("%lf%lf",&p1.x,&p1.y);
        for(int i=2;i<n;i++){
            scanf("%lf%lf",&p2.x,&p2.y);
            double area=Area(p0,p1,p2);
            sum_area+=area;
            sum_x+=(p0.x+p1.x+p2.x)*area;
            sum_y+=(p0.y+p1.y+p2.y)*area;
            p1=p2;
        }
        printf("%.2lf %.2lf\n",sum_x/sum_area/3,sum_y/sum_area/3);
    }
    return 0;
}