hdu2899 Strange fuction 三分搜索

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.

题解：函数是凸性的，用二分搜索需要做特殊处理，求解凸性函数用三分搜索是比较方便的。

类似二分的定义Left和Right，mid = (Left + Right) / 2，midmid = (mid + Right) / 2; 

如果mid靠近极值点，则Right = midmid；否则(即midmid靠近极值点)，则Left = mid;

对于求最大极值，if(sum_mid>sum_midmid)
                r=midmid-1e-8;

对于求最小极值 ，if(sum_mid<sum_midmid)
                r=midmid-1e-8;

#include<cstdlib>
#include<cstdio>
#include<iostream>
#include<cmath>
#include<cstring>
#include<algorithm>
#define LL long long
#define maxn 10001
#define INF 2147483646
using namespace std;
#define eps 1e-7
double mid_pow(double u,double y)
{
    return (6*pow(u,7.0)+8*pow(u,6.0)+7*pow(u,3.0)+5*pow(u,2.0)-y*u);
}
int main()
{
    int T;
    double y,l,r,x,mid,midmid;
    cin>>T;
    while(T--)
    {
        cin>>y;
        l=0,r=100;
        while(r-l>eps)
        {
            mid=(l+r)/2.0;
            midmid=(mid+r)/2.0;
            double sum_mid=mid_pow(mid,y);
            double sum_midmid=mid_pow(midmid,y);
            if(sum_mid<sum_midmid)//求解最小极值
                r=midmid-1e-8;
            else
                l=mid-1e-8;
        }
        printf("%.4lf\n",mid_pow(midmid,y));
    }
}