F(x) = 6 * x^7+8*x^6+7*x^3+5*x^2-y*x (0 <= x <=100)
求最小值
導數求導，導數是一個遞增函式，當F’(x0)=0,求x0,該點是極值點也是最小值點。用二分找該點，浮點數的二分。

double eps = 1e-6;
while(right-left>=eps){ 
    mid=(left+right)/2;
    double temp=der(mid);
    if(temp<y)
        left=mid;
    else
                right=mid;
}

#include <iostream>
 

#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <math.h>
#define ll long long
using namespace std;
double eps = 1e-6;
double x,y;

double der(double dx){
    return 42*pow(dx,6)+48*pow(dx,5)+21*pow(dx,2)+10*dx;
}

int main()
{
    int m;
    while(~scanf("%d"
 
,&m)){
        while(m--){
            scanf("%lf",&y);
            double left=0,right=100,mid;
            while(right-left>=eps){
                mid=(left+right)/2;
                double temp=der(mid);
                if(temp<y)
                    left=mid;
                else
                    right=mid;
            }
            double
 
 ans=6*pow(mid,7)+8*pow(mid,6)+7*pow(mid,3)+5*pow(mid,2)-y*mid;
            printf("%.4f\n",ans);
        }
    }
    return 0;
}