C++ :

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
double D,H,h;
double decide(double x){return D-x+H-(H-h)*D/x;}
int main()
{
	int t;scanf("%d",&t);
	while(t--)
	{
		scanf("%lf%lf%lf",&H,&h,&D);//H:灯泡高度,h:身高,D:灯泡和墙水平距离 
		double l=D-h*D/H,r=D;
		while(r-l>1e-8)
		{
			double lmid=l+(r-l)/3;
			double rmid=r-(r-l)/3;
			if(decide(lmid)>decide(rmid))r=rmid;
			else l=lmid;
		}
		printf("%.3lf\n",decide(l));
	}
	return 0;
}