http://poj.org/problem?id=1905

#include<iostream>
#include<string>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<algorithm>
using namespace std;
const double K=1e-10;
const double PI=acos(-1);
int main()
{
??? double L,n,C;
??? while(cin>>L>>n>>C)
??? {
??????? if(L<0&&n<0&&C<0)
??????? break;
??????? double L1=(1+n*C)*L;
??????? double high=PI;
??????? double low=0;
??????? double mid=(high+low)/2;
??????? while(high-low>K)
??????? {
??????????? //cout<<(mid/2)<<" "<<sin(mid/2)<<endl;
??????????? //cout<<L1<<" "<<(mid*(L/2*asin(mid/2)))<<endl;
??????????? if(L1-(mid*(L/2/sin(mid/2)))>=K)
??????????? {
??????????????? low=mid;
??????????? }
??????????? else
??????????? {
??????????????? high=mid;
??????????? }
??????????? mid=(high+low)/2;
??????? }
??????? //cout<<mid<<endl;
?????? printf("%.3f\n",(L/2/sin(mid/2)-L/2/tan(mid/2)));
??? }
??? return 0;
}

poj 1905 Expanding Rods