hdu 4033

现在还不怎么适应比赛的节奏啊，二分边长即可，余弦定理求角度即可，注意，要先判断所选取的边是否合法

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#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
const double  pi = acos(-1.0);
const double expn=1e-8;
double a[110];
int n;
double get_angel(double x,double y,double z)
{
    return acos((x*x+y*y-z*z)/(2*x*y));
}
int f(double x)
{
    double res=0,a1,a2;
    for (int i=1; i<=n; i++)
    {
        a1=a[i];
        if (i<n) a2=a[i+1]; else a2=a[1];
        if(x-a1-a2>expn) return 1;
        if(fabs(x-a1-a2)<expn) return 1;
        if(fabs(a1-a2)-x>expn) return -1;
        if(fabs(fabs(a1-a2)-x)<expn) return -1;
        res+=get_angel(a1,a2,x);
    }
    if (fabs(res-pi*2)<expn) return 0;
    if ((res - pi*2) > expn) return 1; else return -1;
}
double bsearch(double l,double r)
{
    double mid;    
    for(int k=1;k<100;k++)
    {
        mid=(l+r)/2;
        int t=f(mid);
        if(t==0) return mid;
        if(t>0) r=mid;
        else l=mid;
    }
    return -1;
}
int main()
{
    int t,cases=1,i;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d",&n);
        double max=0;
        for(i=1;i<=n;i++)
        {
            scanf("%lf",&a[i]);
            if(a[i]>max)
                max=a[i];
        }
       double ans=bsearch(0,2*max);
       if(ans<0) printf("Case %d: impossible\n",cases++);
       else printf("Case %d: %.3lf\n",cases++,ans);
    }
    return 0;
}