double dis(Point a)
{
return a.x*a.x+a.y*a.y;
}
struct Circle Circumcircle()
{
Circle tmp;
double a,b,c,c1,c2;
double xa,ya,xb,yb,xc,yc;
a = sqrt(dis(p[3]-p[1]));
b = sqrt(dis(p[1]-p[2]));
c = sqrt(dis(p[2]-p[3]));
//根据s = a*b*c/R/4,求半径
tmp.r = (a*b*c)/(area()*4.0);
xa = p[3].x;
ya = p[3].y;
xb = p[1].x;
yb = p[1].y;
xc = p[2].x;
yc = p[2].y;
c1 = (dis(p[3])-dis(p[1]))/2;
c2 = (dis(p[3])-dis(p[2]))/2;
tmp.center.x = (c1*(ya-yc)-c2*(ya-yb))/((xa-xb)*(ya-yc)-(xa-xc)*(ya-yb));
tmp.center.y = (c1*(xa-xc)-c2*(xa-xb))/((ya-yb)*(xa-xc)-(ya-yc)*(xa-xb));
return tmp;
}