三角形外心

double x1,Y1,x2,y2,x3,y3;
struct vec
{
    double a,b,c;
    vec(double x,double y,double z):a(x),b(y),c(z){};
};
vec pro(double x1,double Y1,double x2,double y2)
{
    return (vec){x2-x1,y2-Y1,-((x2-x1)*(x2-x1)+(y2-Y1)*(y2-Y1))/2};
}
int main()
{
    while(cin>>x1>>Y1>>x2>>y2>>x3>>y3)
    {
        vec a=pro(x1,Y1,x2,y2),b=pro(x1,Y1,x3,y3);
        double cx=-(b.c*a.b-a.c*b.b)/(b.a*a.b-b.b*a.a);
        double cy=-(b.c*a.a-a.c*b.a)/(b.b*a.a-a.b*b.a);
        double r=sqrt(cx*cx+cy*cy);//半径
        cx+=x1,cy+=Y1;//外心横坐标，纵坐标
        double c=-2*cx,d=-2*cy,e=cx*cx+cy*cy-r*r;//圆一般方程参数
    }
    return 0;
}