POJ 1905（expanding rods)

#include<iostream>
#include<math.h>
#include<iomanip>
using namespace std;
#define esp  1e-7
int main()
{
 double l,d,c;
 while(cin>>l>>d>>c)
 {
     if(l<0&&d<0&&c<0)
        break;
     if(l==0||c==0||d==0)
        cout<<"0.000"<<endl;
     else 
     {
        double high=l*0.5;
        double low=0.0;
        double s=(1+d*c)*l;
        double mid;
        while(high-low>esp)
        {
             mid=(high+low)/2;
            double  r=(4*mid*mid+l*l)/(8*mid);
            double  s1=2*r*asin(l/(2*r));
            if(s1<s) low=mid;
            else high=mid;
        }
        double h=mid;
        cout<<fixed<<setprecision(3)<<h<<endl; 
     }
 }
 return 0;
}