#include<iostream>
#include<cmath>
#include<iomanip>
using namespace std;
int main(){
	int t;
	cin>>t;
	cout<<fixed<<setprecision(2);
	while(t--){
		double a,b,c,d,s,ar;
		cin>>a>>b>>c>>d;
		s=(double)((double)(a+b+c+d)/(double)2);
		ar=(double)(s-a)*(s-b)*(s-c)*(s-d);
		cout<<(double)sqrt(ar)<<endl;
	}
	return 0;
}