//ignore header files
int main(int argc, char const* argv[])
{
    int n;
    scanf("%d",&n);
    double s[10005];
    while(n--){
        int m;
        scanf("%d",&m);
        for(int i = 0;i<m;i++){
            double e;
            scanf("%lf",&e);
            s[i] = e;
        }
        sort(s,s+m);
        double max = 0; 
        for(int i = m-1;i >= 2;i--){
            double a = s[i];
            double  b = s[i-1];
            double c = s[i-2];
            if(a+b<c || a+c<b || b+c<a)
                continue;
            double p = (a+b+c)/2;
            double s = p*(p-a)*(p-b)*(p-c);
            if (s>max){
                max = s;
            }
        }
        printf("%.2lf\n",sqrt(max));
    }
    return 0;
}