POJ 2242

 #include <iostream>
 #include <cmath>
 #include <iomanip>
 using namespace std;
 #define PI 3.141592653589793

 struct node
 {
     double x;
     double y;
 };

 node p_1;
 node p_2;
 node p_3;

 double a;
 double b;
 double c;
 double give_len(node a,node b);

 int main()
 {
     //freopen("acm.acm","r",stdin);
     double cos_c;
     double sin_c;
     double s;
     while(cin>>p_1.x>>p_1.y>>p_2.x>>p_2.y>>p_3.x>>p_3.y)
     {
         a = give_len(p_1,p_2);
         b = give_len(p_1,p_3);
         c = give_len(p_2,p_3);
     //    cout<<"--- a"<<a<<endl;
     //    cout<<b<<endl;
     //    cout<<c<<endl;
         cos_c = (a*a + b*b - c*c)/(2.0*a*b);
         sin_c = sqrt(( - cos_c*cos_c));
         s = 0.5 * a*b*sin_c;
         cout<<setiosflags(ios::fixed)<<setprecision()<<a*b*c*PI/(s*2.0)<<endl;
     }
 }

 double give_len(node a,node b)
 {
     return sqrt( (a.x-b.x)*(a.x-b.x) + (a.y - b.y)*(a.y - b.y) );
 }