hdu 4033 2011成都赛区网络赛 余弦定理+二分 **

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<algorithm>
 4 #include<cstring>
 5 #include<cmath>
 6 #include<queue>
 7 #include<map>
 8 using namespace std;
 9 #define MOD 1000000007
10 const int INF=0x3f3f3f3f;
11 typedef long long ll;
12 #define cl(a) memset(a,0,sizeof(a))
13 #define ts printf("*****\n");
14 const int MAXN=1005;
15 int n,m,tt;
16 const double eps = 1e-8;
17 const double pi = acos(-1.0);
18 double a[MAXN];
19 int main()
20 {
21     int i,j,k;
22     #ifndef ONLINE_JUDGE
23     freopen("1.in","r",stdin);
24     #endif
25     scanf("%d",&tt);
26     int ca=1;
27     while(tt--)
28     {
29         scanf("%d",&n);
30         for(i=0;i<n;i++)
31         {
32             scanf("%lf",&a[i]);
33         }
34         a[n]=a[0];
35         bool flag=0;
36         double l=0,r=20000;
37         double mid;
38         for(i=1;i<=n;i++)
39         {
40             r = min(r,a[i]+a[i-1]);
41             l = max(l,fabs(a[i]-a[i-1]));
42         }
43         while(r-l>eps)
44         {
45             mid=(l+r)/2.0;
46             double s,sum=0;
47             for(i=1;i<=n;i++)
48             {
49                 s=(a[i]*a[i]+a[i-1]*a[i-1]-mid*mid)/(2.0*a[i]*a[i-1]);
50                 sum+=acos(s);
51             }
52             if(fabs(sum-2*pi)<eps)
53             {
54                 flag=1;
55                 break;
56             }
57             else if(sum>2*pi)
58             {
59                 r=mid;
60             }
61             else l=mid;
62         }
63         printf("Case %d: ",ca++);
64         if(flag)
65             printf("%.3f\n",mid);
66         else
67             printf("impossible\n");
68     }
69 }