Area

http://poj.org/problem?id=1265

 #include<cstdio>
 #include<istream>
 #include<algorithm>
 #include<cmath>
 int n;
 using namespace std;
 const int maxn=;
 const double pi=acos(-1.0);
 int gcd(int a,int b)
 {
     return b==?a:gcd(b,a%b);
 }
 struct Point  //点的定义
 {
     double x,y;
     Point() {}
     Point (double a,double b):x(a),y(b) {}
 };
 Point a[maxn],b[maxn];
 double del(const Point &a,const Point &b)
 {
     return(a.x*b.y-a.y*b.x);
 }
 int main()
 {
     int t,c=;

     scanf("%d",&t);
     while(t--)
     {
         c++;
         scanf("%d",&n);
         int xx=,yy=,x,y,num=;
         for(int i=; i<n; i++){
             scanf("%d%d",&x,&y);
             num+=gcd(abs(x),abs(y));
             b[i+].x=xx+x;
             b[i+].y=yy+y;
             xx=b[i+].x;
             yy=b[i+].y;
         }
         double sum=;
         b[].x=;
         b[].y=;
         for(int i=; i<n; i++)
           sum+=del(b[i],b[i+]);
         printf("Scenario #%d:\n",c);
         printf("%d %d %.1lf\n",int(sum/2.0)+-(num/),num,sum/);
         printf("\n");
     }
 }