三分模板

#include <cstdio>
#include <cstring>
#include <cmath>
#define eps 1e-7

double a[20];
int N;
double l,r,ans;

inline double f(double x){
    double sum = a[N];
    for(register int i=N-1;i>=0;--i){
        sum = sum*x + a[i];
    }
    return sum;
}
int main(){

    scanf("%d%lf%lf",&N,&l,&r);
    int x;
    for(register int i=N;i>=0;--i){
        scanf("%lf",&a[i]);
    }

    while(fabs(r-l)>eps){
        double mid_1 = l+(r-l)/3.0000001;
        double mid_2 = r-(r-l)/3.0000001;
        if(f(mid_1)>f(mid_2))r = mid_2;
        else l = mid_1;
    }
    printf("%.5lf",l);
    return 0;
}