几何模版（求圆心坐标，半径，反正切值确定线段方位）

double chaji(double ax,double ay,double bx,double by,double cx,double cy)    //两条线段斜率的差，double !!!
{
    return (by-ay)*(cx-ax)-(cy-ay)*(bx-ax);
}
void count(double x1,double y1,double x2,double y2,double x3,double y3)
{
    double a,b,c,d,e,f;
    a=2*(x2-x1);
    b=2*(y2-y1);
    c=x2*x2+y2*y2-x1*x1-y1*y1;
    d=2*(x3-x2);
    e=2*(y3-y2);
    f=x3*x3+y3*y3-x2*x2-y2*y2;
    x=(b*f-e*c)/(b*d-e*a);
    y=(d*c-a*f)/(b*d-e*a);
    r=sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1));

}
double bj(double xp,double yp,double x1,double y1)
{
    double s=sqrt((xp-x1)*(xp-x1)+(yp-y1)*(yp-y1));
    return s;
}
double nodecount(double x1,double y1,double x2,double y2,double x3,double y3,double xp,double yp)
{
    double f1=atan2(x1-x,y1-y);
    double f2=atan2(x2-x,y2-y);
    double f3=atan2(x3-x,y3-y);
    double f4=atan2(xp-x,yp-y);//atans(x,y)反切值函数，见c++函数应用，此时所求为(xp,yp)以(x,y)为原点的反切值

    double ans1=fabs(r-bj(xp,yp,x,y));//fabs绝对值函数