POJ 1905 Expanding Rods 二分答案几何

题目：http://poj.org/problem?id=1905

恶心死了，POJ的输出一会要lf，一会要f，而且精度1e-13才过，1e-12都不行，错了一万遍终于对了。

#include <stdio.h>
#include <math.h>

int main()
{
    double l, n, c, r;
    while(scanf("%lf %lf %lf", &l, &n, &c) != EOF)
    {
        if(l < 0 && n < 0 && c < 0)break;
        double arc = (1 + n*c) * l;
        double low = 0, high = acos(-1.0) / 2;
        while(high-low > 1e-13)
        {
            double mid = (high+low) / 2;
            r = l/2 / sin(mid);
            if(r * mid < arc / 2)
            {
                low = mid;
            }
            else
            {
                high = mid;
            }
        }
        printf("%.3lf
", r-r*cos(low));
    }
    return 0;
}