1 #include<stdio.h>
2 #include<string.h>
3 #include<cmath>
4 #include<algorithm>
5 #define pi 3.1415926535897932384626433832795
6 using namespace std;
7 double area(double x1,double y1,double r1,double x2,double y2,double r2)
8 {
9 double d=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
10 double r=r1<r2?r1:r2;
11 if (d>=r1+r2) return 0;
12 if (d<=fabs(r1-r2)) return pi*r*r;
13 double a1=acos((r1*r1+d*d-r2*r2)/(2.0*r1*d));
14 double a2=acos((r2*r2+d*d-r1*r1)/(2.0*r2*d));
15 double ans=0.0;
16 ans-=r1*sin(a1)*d;
17 ans+=r1*r1*a1+r2*r2*a2;
18 return ans;
19 }
20 int main()
21 {
22 double x1,y1,r1,x2,y2,r2;
23 while (~scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&r1,&x2,&y2,&r2))
24 printf("%.3lf\n",area(x1,y1,r1,x2,y2,r2));
25 return 0;
26 }
题目：http://poj.org/problem?id=2546
