poj 1905 Expanding Rods 二分

/**
题解晚上写
**/
#include <iostream>
#include <math.h>
#include <algorithm>
#include <cstdio>
using namespace std;
const double esp = 1e-5;
int main()
{
    double l,n,c;
    while(cin>>l>>n>>c){
        if(l<0&&n<0&&c<0)
            break;
        double ll;
        ll = (1+n*c)*l;
        double low,high;
        low =0;
        high= l/2;
        double r,mid;
        while(high-low>esp){
            mid = (high+low)/2.0;
            r = ((mid*mid*4)+(l*l))/(8*mid);
            if(2*r*asin(l/(2*r))<ll)
                low = mid;
            else
                high = mid;
        }
        printf("%.3lf\n",high);
    }
    return 0;
}