poj 1905 图形推算+二分

#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<math.h>
#include<string.h>
#include<iomanip>
using namespace std;
//const double esp=1e-5;
int main()
{
    double L,n,c,s,r,h;
    while(~scanf("%lf %lf %lf",&L,&n,&c))
    {
        if(L<0&&n<0&&c<0)
            break;
          //
        s=L*(1+n*c);
        double low=0.0;
        double high=L*0.5;
        double mid;// printf("%lf\n",s);
        while(high-low>0.00001)
        {
            mid=(high+low)/2;
            r=(4*mid*mid+L*L)/(8*mid);
            if(2*r*asin(L/(2*r))<s)
              low=mid;
            else
              high=mid;
        }
        h=mid;
        cout<<fixed<<setprecision(3)<<h<<endl;
      //printf("%.3f\n",mid);
    }
}