简单题，二分就行；
1 #include<cstdio> 2 #include<cmath> 3 #define pi acos(-1.0) 4 #define eps 0.000001 5 #define maxn 10009 6 using namespace std; 7 double area[maxn]; 8 9 int main() 10 { 11 int t,n,f,ri; 12 double r=-1; 13 scanf("%d",&t); 14 while(t--) 15 { 16 scanf("%d%d",&n,&f); 17 f=f+1; 18 for(int i=0;i<n;i++) 19 { 20 scanf("%d",&ri); 21 area[i]=ri*ri*pi; 22 if(area[i]>r)r=area[i]; 23 } 24 double l=0.0; 25 while(r-l>eps) 26 { 27 double mid=(r+l)/2; 28 int cnt=0; 29 for(int i=0;i<n;i++) 30 cnt+=floor(area[i]/mid); 31 if(cnt>=f)l=mid; 32 else r=mid; 33 } 34 printf("%.4lf ",l); 35 } 36 return 0; 37 }