由题知道所求的值0<=x<l/2；所以用二分就可以很快的解决！！！
代码如下：
1 #include<iostream> 2 #include<stdio.h> 3 #include<algorithm> 4 #include<iomanip> 5 #include<cmath> 6 #include<cstring> 7 #include<vector> 8 //#define ll __int64 9 #define pi acos(-1.0) 10 #define MAX 50000 11 using namespace std; 12 int main(){ 13 int n,m,i,sum; 14 double l2,c,l,ll,rr,r,mid,s; 15 while(cin>>l>>n>>c){ 16 if (l<0&&n<0&&c<0) break; 17 l2 = (1+c*n)*l; 18 ll=0,rr=l/2; 19 while(rr-ll>1e-8){ 20 mid = (ll+rr)/2; 21 r = mid/2+l*l/(8*mid); 22 s = 2*r*asin(l/(2*r)); 23 if (fabs(l2-s)<=1e-8) 24 break; 25 else if (l2>s) ll = mid; 26 else rr = mid; 27 } 28 printf("%.3lf ",mid); 29 } 30 return 0; 31 }