hdu 4033 2011成都赛区网络赛 余弦定理+二分 **

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<queue>
#include<map>
using namespace std;
#define MOD 1000000007
const int INF=0x3f3f3f3f;
typedef long long ll;
#define cl(a) memset(a,0,sizeof(a))
#define ts printf("*****\n");
const int MAXN=1005;
int n,m,tt;
const double eps = 1e-8;
const double pi = acos(-1.0);
double a[MAXN];
int main()
{
int i,j,k;
#ifndef ONLINE_JUDGE
freopen("1.in","r",stdin);
#endif
scanf("%d",&tt);
int ca=1;
while(tt--)
{
scanf("%d",&n);
for(i=0;i<n;i++)
{
scanf("%lf",&a[i]);
}
a
=a[0];
bool flag=0;
double l=0,r=20000;
double mid;
for(i=1;i<=n;i++)
{
r = min(r,a[i]+a[i-1]);
l = max(l,fabs(a[i]-a[i-1]));
}
while(r-l>eps)
{
mid=(l+r)/2.0;
double s,sum=0;
for(i=1;i<=n;i++)
{
s=(a[i]*a[i]+a[i-1]*a[i-1]-mid*mid)/(2.0*a[i]*a[i-1]);
sum+=acos(s);
}
if(fabs(sum-2*pi)<eps)
{
flag=1;
break;
}
else if(sum>2*pi)
{
r=mid;
}
else l=mid;
}
printf("Case %d: ",ca++);
if(flag)
printf("%.3f\n",mid);
else
printf("impossible\n");
}
}