练习2- Strange fuction

题目：

Problem Description

Now, here is a fuction:<br>&nbsp;&nbsp;F(x) = 6 * x^7+8*x^6+7*x^3+5*x^2-y*x (0 &lt;= x &lt;=100)<br>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<br>100<br>200

 

Sample Output

-74.4291<br>-178.8534

 

Author

Redow

题意：

           解方程；

思路：

           解方程，对方程进行求导，然后跟第一题做法一样，求出x;

代码：

          

# include <iostream># include <cmath># include <iomanip>using namespace std;double f (double x, double y){    return 6 * pow(x, 7) + 8 * pow(x, 6) + 7 * pow(x, 3)+ 5 * x * x - y * x;}double ff (double x){    return 42 * pow(x, 6) + 48 * pow(x, 5) + 21 * x * x + 10 * x ;}int main(){    int n;    cin >> n;    while (n--)    {        double min = 0, max = 100, x;        int y;        cin >> y;        while (1)        {            x = (min + max) / 2;            if (fabs(ff(x) - y) < 1e-4)            {                cout << fixed << setprecision(4) <<  f (x, y) << endl;                break;            }            else if (ff(x) > y)            {                max = x;            }            else            {                min = x;            }        }    }    return 0;}    //42 * pow(x, 6) + 48 * pow(x, 5) + 10 * x - y;