题意：
给两个圆，求它们的面积交。
分析：
海伦公式，余弦定理等可解。
代码：
//poj 2546
//sep9
#include <iostream>
#include <cmath>
using namespace std;
const double pi=acos(-1.0);int main()
{double x1,y1,r1,x2,y2,r2;scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&r1,&x2,&y2,&r2);double d=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));if(d>=r1+r2){printf("%.3lf",0);return 0;}if(d<=max(r1,r2)-min(r1,r2)){printf("%.3lf",pi*min(r1,r2)*min(r1,r2));return 0;}double p=(r1+r2+d)/2;double s=2*sqrt(p*(p-r1)*(p-r2)*(p-d));double A1=2*acos((r1*r1+d*d-r2*r2)/(2*r1*d));double A2=2*acos((r2*r2+d*d-r1*r1)/(2*r2*d));double s1=A1*r1*r1/2;double s2=A2*r2*r2/2;printf("%.3lf",s1+s2-s+1e-9);return 0;
}