zoj-3207（三分）

//关系式分两个
//令x = l(H->h), 即灯泡到人的水平距离
//当x 在 [0, d - d * h / H]范围内，ans 递增有最大值为 ans = d * h / H;
//当x 在 [d - d * h / H, d]范围内，有关系式ans = H + d - (x + d * (H - h) / x);
//三分法求极值
int main() {
	int t;
	double h, H, d;
	scanf("%d", &t);
	while (t--) {
		scanf("%lf %lf %lf", &H, &h, &d);
		double ans = d * h / H;
		double l = d - ans, r = d;
		double mul = d * (H - h);
		int cas = 10000;
		while (cas--) {
			double lmid = (2 * l + r) / 3;
			double rmid = (l + 2 * r) / 3;

			double ans1 = lmid + mul / lmid;
			double ans2 = rmid + mul / rmid;

			if (ans1 < ans2)
				r = rmid;
			else
				l = lmid;
		}

		ans = max(ans, H + d - l - mul / l);
		printf("%.3lf\n", ans);
	}
}