三分

ref:http://blog.csdn.net/acdreamers/article/details/9989197

ref:http://blog.csdn.net/pi9nc/article/details/9666627

Code(luogu 3382):

#include<cstdio>
#include<cstring>
#include<algorithm>
#define MN 15
#define eps 1e-6
using namespace std;
inline int in(){
    int x=0;bool f=0; char c;
    for (;(c=getchar())<'0'||c>'9';f=c=='-');
    for (x=c-'0';(c=getchar())>='0'&&c<='9';x=(x<<3)+(x<<1)+c-'0');
    return f?-x:x;
}
double a[MN],l,r;
int n;
inline double check(double x){
    double res=0.0;
    for (int i=0;i<=n;++i) res=res*x+a[i];
    return res;
}
int main()
{
    scanf("%d%lf%lf",&n,&l,&r);
    for (int i=0;i<=n;++i) scanf("%lf",&a[i]);
    while (r-l>eps){
        double lmid=l+(r-l)/3,rmid=r-(r-l)/3;
        if (check(lmid)<check(rmid)) l=lmid;
        else r=rmid;
    }printf("%.5lf",l);return 0;
}