鏈接：http://acm.split.hdu.edu.cn/showproblem.php?pid=5858

題意：知道正方形邊長，求陰影部分面積

題解：直接求出兩圓相交部分面積。

CODE：

#include <bits/stdc++.h>
using namespace std;
#define PI acos(-1)
#define eps 1e-5
double Area_of_overlap(double d,double r1,double r2)
{
    if(r1 + r2 < d + eps)return 0;
    if(d < fabs(r1 - r2) + eps)
    {
        double r = min(r1,r2);
        return PI*r*r;
    }
    double x = (d*d + r1*r1 - r2*r2)/(2*d);
    double t1 = acos(x / r1);
    double t2 = acos((d - x)/r2);
    return r1*r1*t1 + r2*r2*t2 - d*r1*sin(t1);
}
int main()
{
    int t;
    double l;
    double d;
    double r1,r2;
    double ans;
    double s;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%lf",&l);
        d=sqrt(2.0)/2.0*l;
        r1=l;
        r2=l/2.0;
        s=PI*r2*r2;
        ans=Area_of_overlap(d,r1,r2);
        ans=2*(s-ans);
        printf("%.2lf\n",ans);
    }
    return 0;
}