浮点---二分

while(r-l>=esp){  
        double mid=(r+l)/2;  
        if(check(mid)){  
            l=mid;  
            ans=mid;  
        }  
        else r=mid;  
    } 

#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const double PI=3.14159265359;
const int N=1E4+7;
double arr[N];
double esp=1e-5;
int n,f;//n个蛋糕，分成f份。
int check(double x){
    int sum=0;
    for(int i=1;i<=n;i++){
        sum+=int (arr[i]/x);
    }
    if(sum>=f) return 1;
    return 0;
}
void solve(){
    scanf("%d%d",&n,&f);
    f++;
    double c=N;
    double r=0;
    for(int i=1;i<=n;i++){
        scanf("%lf",&arr[i]);
        arr[i]=arr[i]*arr[i];
        r+=arr[i];
    }
    double l=0,ans=0;
    while(r-l>=esp){
        double mid=(r+l)/2;
        int c=check(mid);
        if(c==1){
            l=mid;
            ans=mid;
        }
        else r=mid;
    }
    printf("%.4f\n",ans*PI);
}
int main()
{
    int t;
    scanf("%d",&t);
    while(t--) solve();
    return 0;
}