hdu 2899 Strange fuction 二分

/*
 * hdu2899.c
 *
 *  Created on: 2011-10-9
 *      Author: bjfuwangzhu
 */

#include<stdio.h>
#include<math.h>
#define eps 1.0e-8
double ff(double x, double y) {
    return 42.0 * pow(x, 6.0) + 48.0 * pow(x, 5.0) + 21.0 * pow(x, 2.0)
            + 10.0 * x - y;
}
double f(double x, double y) {
    return 6.0 * pow(x, 7.0) + 8.0 * pow(x, 6.0) + 7.0 * pow(x, 3.0)
            + 5.0 * pow(x, 2.0) - y * x;
}
double bfsearch(double y) {
    double left, right, mid;
    left = 0.0, right = 100.0;
    while (fabs(left - right) > eps) {
        mid = (left + right) / 2.0;
        if (ff(mid, y) < 0.0) {
            left = mid;
        } else {
            right = mid;
        }
    }
    return mid;
}
int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    int t;
    double y, x;
    scanf("%d", &t);
    while (t--) {
        scanf("%lf", &y);
        x = bfsearch(y);
        printf("%.4lf\n", f(x, y));
    }
    return 0;
}