51nod 1629 B君的圆锥

推推就出来了

#include <bits/stdc++.h>
using namespace std;

int main()
{
    double s;
    double pi = acos(-1);
    cin >> s;
    double r2 = s/(4*pi);
    double ninev2 = s*s*r2-2*s*pi*r2*r2;
    cout << fixed << setprecision(6) << sqrt(ninev2/9) << endl;
    return 0;
}