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((1 - cos_c*cos_c));
        s = 0.5 * a*b*sin_c;
        cout<<setiosflags(ios::fixed)<<setprecision(2)<<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) );
}