Strange fuction

Problem Description

Now, here is a fuction:
  F(x) = 6 * x^7+8*x^6+7*x^3+5*x^2-y*x (0 <= x <=100)
Can you find the minimum value when x is between 0 and 100.

Input

The first line of the input contains an integer T(1<=T<=100) which means the number of test cases. Then T lines follow, each line has only one real numbers Y.(0 < Y <1e10)

Output

Just the minimum value (accurate up to 4 decimal places),when x is between 0 and 100.

Sample Input

2
100
200

Sample Output

-74.4291

-178.8534

題意：找出那個函數在x屬於0~100內的最小值。

思路：首先想到的應該是求導求駐點，然後用二分法逼近。

#include "cstdio"
#include "cstring"
#include "cmath"
#define mi 0.00001
using namespace std;
double x,y;
double f(double x)
{
    return 42*pow(x,6)+48*pow(x,5)+21*pow(x,2)+10*x-y;
}
double fun(double x){
    return 6*pow(x,7)+8*pow(x,6)+7*pow(x,3)+5*pow(x,2)-x*y;
}
int main()
{
    int T;

    scanf("%d",&T);
    while(T--)
    {
        scanf("%lf",&y);
        double l=0,r=100;
        double min;
        double mid=(l+r)/2;
        while(fabs(f(mid))>mi)
        {
            mid=(l+r)/2;
            double a=f(mid);
            if(a>=0)
            {
                r=mid;
            }
            else if(a<0)
            {
                l=mid;
            }
        }
        printf("%.4lf\n",fun(mid));
    }
    return 0;
}