精度限定到1e-12即可。

 

 

1 #include 
      
        2 #include 
       
         3  4 using namespace std; 5  6 const double pi = 3.14159265359; 7 const double ep = 1e-12; 8  9 double l, n, c;10 11 void solve(){12     double low = ep, high = pi / 2;13     double l1 = (1 + n * c) * l;14     double d1, d2, mid;15     d2 = l / l1;16     while(high - low > ep){17         mid = (low + high) / 2;18         d1 = sin(mid) / mid;19         if(d1 > d2) low = mid;20         else high = mid;21     }22     d1 = sin(low), d2 = sin(low / 2);23     printf("%.3f\n", l * d2 * d2 / d1);24 }25 26 int main(){27     //freopen("in.txt", "r", stdin);28     while(~scanf("%lf%lf%lf", &l, &n, &c) && (l + 1)){29         solve();30     }31     return 0;32 }