题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=3714

懂了三分思想和F(x)函数的单调性质，这题也就是水题了

#include "stdio.h"  //最后得到的F(x)函数要么是单调的，要么是先减后增的，有了这个性质，就可以三分了~

#include "string.h"

#define N 10005

#define eps 1e-9

#define INF 0x3fffffff

struct node

{

    int a;

    int b;

    int c;

}q[N];

int n;

double S(double x)

{

    double sx;

    double maxt = -INF;

    for(int i=0; i<n; ++i)

    {

        sx = q[i].a*x*x+q[i].b*x+q[i].c;

        if(sx>maxt)

            maxt = sx;

    }

    return maxt;

}

int main()

{

    int i,T;

    double ans_mid,ans_mid_mid;

    double left,right,mid,mid_mid;

    scanf("%d",&T);

    while(T--)

    {

        scanf("%d",&n);

        for(i=0; i<n; ++i)

            scanf("%d %d %d",&q[i].a,&q[i].b,&q[i].c);

        left = 0;

        right = 1000;

        while(right-left>eps)

        {

            mid = (right+left)/2;

            ans_mid = S(mid);

            mid_mid = (mid+right)/2;

            ans_mid_mid = S(mid_mid);

            if(ans_mid>ans_mid_mid)

                left = mid;

            else

                right=  mid_mid;

        }

        printf("%.4lf\n",S(right));

    }

}