題目地址：http://acm.nyist.net/JudgeOnline/problem.php?pid=67

思路：任意2邊向量的叉積的絕對值的1/2即為三角形的面積

AC程式碼：

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <map>
#include <cstring>
#include <climits>
#include <cmath>
#include <cctype>

const int inf = 0x7f7f7f7f;//2139062143
typedef long long ll;
using namespace std;

int main()
{
    double x1,x2,x3,y1,y2,y3;
    while(scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3))
    {
        if(x1+x2+x3+y1+y2+y3 == 0)
            break;
        double sum = fabs((x2-x1)*(y3-y1)-(x3-x1)*(y2-y1)) / 2;
        printf("%.1lf\n",sum);
    }
    return 0;
}