poj 1905 Expanding Rods

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\n",mid);

29     }

30     return 0;

31 }

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