(Miku)[https://loj.ac/problem/10013]

画个图就知道，f(x)还是个单峰函数

三分起

#include<iostream>
#include<cstdio> 
#include<algorithm>
#include<cmath>
using namespace std;
double xz=1e-9;
int t,n;
double l,r;
double lm,rm;
double a[100005],b[100005],c[100005];
double ans;
double cou(double x){
    ans=-0x7fffffff;
    for(int i=1;i<=n;++i){
        ans=max(ans,a[i]*x*x+b[i]*x+c[i]);
    }
    return ans;
}
double sf(double l,double r){
    while(l+xz<r){
        lm=l+(r-l)*0.3;
        rm=l+(r-l)*0.7;
        if(cou(lm)<cou(rm)){
            r=rm;
        }else{
            l=lm;
        }   
    }
    return cou(l);
}
int main(){
    scanf("%d",&t);
    for(int i=1;i<=t;++i){
        scanf("%d",&n);
        for(int j=1;j<=n;++j){
            scanf("%lf%lf%lf",&a[j],&b[j],&c[j]);
        }
        printf("%.4lfn",sf(0.0,1000.0));
    }
    return 0;
}