题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=5017

题意：略。

思路：知道(x,y)以后可以解方程求得z，然后直接上模拟退火就可以过了。

#include<bits/stdc++.h>
#define MP make_pair
#define PB emplace_back
using namespace std;
typedef long long ll;
template<typename T>
inline T read(T&x){
    x=0;int f=0;char ch=getchar();
    while (ch<'0' || ch>'9') f|=(ch=='-'),ch=getchar();
    while (ch>='0'&& ch<='9') x=x*10+ch-'0',ch=getchar();
    return x=f?-x:x;
}
const double eps=1e-14;
const double INF=2000000000000000;
int i;
double ansx,ansy,ansz,a,b,c,d,e,f;
double dist(double x,double y,double z){return sqrt(x*x+y*y+z*z);}
double get(double x,double y){
    double A=c;
    double B=d*y+e*x;
    double C=a*x*x+b*y*y+f*x*y-1.0;
    double detla=B*B-4.0*A*C;
    if (detla<0) return INF;
    double x1=(-B+sqrt(detla))/(2.0*A);
    double x2=(-B-sqrt(detla))/(2.0*A);
    return dist(x,y,x1)>dist(x,y,x2)?x2:x1;
}
void SA(){
    double T=10000;
    double cx=ansx,cy=ansy,cz=ansz;
    while (T>eps){
        double nx=cx+(rand()*2-RAND_MAX)*T;
        double ny=cy+(rand()*2-RAND_MAX)*T;
        double nz=get(nx,ny);
        if (nz==INF){
            T=T*0.99;
            continue;
        }
        double detla=dist(nx,ny,nz)-dist(cx,cy,cz);
        if (detla<0) ansx=cx=nx,ansy=cy=ny,ansz=cz=nz;
        else if (exp(-detla/T)*RAND_MAX>rand()) cx=nx,cy=ny,cz=nz;
        T=T*0.99;
    }
}
int main(){
    srand(19260817);
    while (~scanf("%lf%lf%lf%lf%lf%lf",&a,&b,&c,&d,&e,&f)){
        ansx=ansy=0;
        ansz=sqrt(1.0/c);
        SA();
        printf("%.7f\n",dist(ansx,ansy,ansz));
    }
    return 0;
}