HDU - 2899 Strange fuction

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 double eps = 1e-6;
 4 double f(double x)
 5 {
 6    return (42 * pow(x, 6) + 48 * pow(x, 5) + 21 * pow(x, 2) + 10 * x);
 7 }
 8 double F(double x, double y)
 9 {
10     return (6 * pow(x, 7) + 8 * pow(x, 6) + 7 * pow(x, 3) + 5 * pow(x, 2) - y * x);
11 }
12 int main()
13 {
14     int T;
15     cin >> T;
16     while(T--){
17         double x, y;
18         cin >> y;
19         double l = 0, r = 100;
20         double mid;
21         while(r - l > eps){
22             mid = (l + r)/2;
23             if(f(mid) < y) l = mid;
24             else r = mid;
25         }
26         cout << fixed << setprecision(4) << F(r, y) << endl;
27     }
28 }