外接圓模板

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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]));    //根據s = a*b*c/R/4，求半徑    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;}