Go: package main

import (
	"fmt"
	"math"
)

func main() {
	var num int
	fmt.Scan(&num)
	for i := 0; i < num; i++ {
		var x1, y1, x2, y2, x3, y3 float64
		fmt.Scanf("%f%f%f%f%f%f", &x1, &y1, &x2, &y2, &x3, &y3)

		a := 2 * (x2 - x1)
		b := 2 * (y2 - y1)
		c := 2 * (x3 - x1)
		d := 2 * (y3 - y1)
		e := x2*x2 - x1*x1 + y2*y2 - y1*y1
		f := x3*x3 - x1*x1 + y3*y3 - y1*y1

		// 直線1: ax + by - e = 0
		// 直線2: cx + dy - f = 0
		px := (d*e - b*f) / (a*d - b*c)
		py := (c*e - a*f) / (b*c - a*d)
		r := math.Sqrt((px-x1)*(px-x1) + (py-y1)*(py-y1))
		fmt.Printf("%.3f %.3f %.3f\n", px, py, r)
	}
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>
using namespace std;
long double ff(long double a){
  
  a*=10000.0;
  //  cout<<a<<endl;
  int c = a;
  if(c%10>4)c+=10;
  c/=10;
  return (long double)c/1000.0;
}



long double ax,ay,bx,by,cx,cy,ox,oy,dx,dy,ex,ey;
//ack はACの傾き  eos はEOの切片
long double abk,ack,dok,eok,eos,dos,eps=0.0;
int n;
int main(){
  // cout<<ff(999.9999)<<endl;
  cin>>n;
  for(int i=0;i<n;i++){
    cin>>ax>>ay>>bx>>by>>cx>>cy;
    //ABの中点をD,  ACの中点をE 外接円の中心をO とする
    dx=(ax+bx)/2.0;
    dy=(ay+by)/2.0;
    ex=(ax+cx)/2.0;
    ey=(ay+cy)/2.0;
    
    if(ax==bx){
      abk=(ay-by)/0.0000000001;
    }else{
      abk=(ay-by)/(ax-bx);
    }
    if(abk==0.0)abk=0.0000000001;
    dok=-1.0/abk;
    dos=dy-dx*dok;
    
    
    if(ax==cx){
      ack=(ay-cy)/0.0000000001;
    }else{
      ack=(ay-cy)/(ax-cx);
    }
    if(ack==0.0)ack=0.0000000001;
    eok=-1.0/ack;
    eos=ey-ex*eok;
    
    //Y  =  dok X  + dos
    //Y  =  eok X  + eos
    //dok X + dos = eok X + eos
    
    long double Y,X;
    if(dok-eok==0.0){
      X=(eos-dos)/0.0000000001;
    }else{
      X=(eos-dos)/(dok-eok);
    }
    Y= dok * X + dos;
    
    printf("%.3Lf %.3Lf ",ff(X),ff(Y));
    //cout<<ax<<' '<<X<<' '<<ay<<' '<<Y<<endl;
    printf("%.3Lf\n" ,  ff( sqrt( pow(ax-X,2) + pow(ay-Y,2) )));

    
    
  }
  return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<math.h>
using namespace std;

int main()
{
	int i,n,flg;
	double x1,x2,x3,y1,y2,y3;
	double m1,m2,xm1,xm2,ym1,ym2,px,py,r;
	//m1,m2…辺の法線 xm1,ym1…辺の中点の座標

	cin>>n;
	for(i=0;i<n;i++){
		flg=0;
		cin>>x1>>y1>>x2>>y2>>x3>>y3;

		xm1=(x1+x2)/2;
		xm2=(x2+x3)/2;
		ym1=(y1+y2)/2;
		ym2=(y2+y3)/2;
		
		if(y1-y2!=0) 		m1=(-1)*(x1-x2)/(y1-y2);
		else if(y1-y2==0)	flg=1;
		
		if(y2-y3!=0)		m2=(-1)*(x2-x3)/(y2-y3);
		else if(y2-y3==0)	flg=2;
		
		if(flg==0) 	px=(m1*xm1-m2*xm2+ym2-ym1)/(m1-m2);
		else if(flg==1)px=xm1;
		else if(flg==2)px=xm2;

		if(flg==1)py=m2*(px-xm2)+ym2;
		else		py=m1*(px-xm1)+ym1;
		r=sqrt((x1-px)*(x1-px)+(y1-py)*(y1-py));
		printf("%.3lf %.3lf %.3lf\n",px,py,r);
	}
}

C++: #include <iostream>
#include <cstdio>
#include <stdio.h>
#include <math.h>
using namespace std;
int main(){
	int n;
	cin>>n;
	for(int i=0;i<n;i++){
		double x1,y1,x2,y2,x3,y3,xp,yp,r;
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		xp=0;
		yp=0;
		double a=y2-y1,b=y3-y1,c=y2-y3,d=x2-x1,e=x3-x1;
		if(y1 != y2 && y1 != y3){
			xp=(c/2-e*(x1+x3)/2/b+d*(x2+x1)/2/a)/(-e/b+d/a);
			yp=-e/b*(xp-(x1+x3)/2)+(y1+y3)/2;
		}else if(y1==y2){
			xp=(x1+x2)/2;
			yp=-e/b*(xp-(x1+x3)/2)+(y1+y3)/2;
		}else if(y1==y3){
			xp=(x1+x3)/2;
			yp=-d/a*(xp-(x1+x2)/2)+(y1+y2)/2;
		}
		r = sqrt((x1-xp)*(x1-xp)+(y1-yp)*(y1-yp));
		
		printf("%.3lf %.3lf %.3lf\n",xp,yp,r);
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>

using namespace std;

int main()
{
    int n;
    cin >> n;
    double x1,y1,x2,y2,x3,y3;
    while(n>0)
    {
        cin >> x1 >> y1 >>x2 >> y2 >> x3 >> y3;
        double s,v1x,v1y,v2x,v2y,h1x,h1y,h2x,h2y;
        v1x = y2-y1;
        v1y = x1-x2;
        v2x = y3-y1;
        v2y = x1-x3;
        h1x = (x2+x1)/2;
        h1y = (y2+y1)/2;
        h2x = (x3+x1)/2;
        h2y = (y3+y1)/2;

        double detA = v1x*v2y-v1y*v2x;
        //double hx = (x2+x1)/2;
        //double hy = (y2+y1)/2;
        s = (v2y*(h2x-h1x) -v2x*(h2y-h1y))/detA;
        double ox,oy;
        ox = h1x + s*v1x;
        oy = h1y + s*v1y;
        if(-0.0005<ox&&ox<0)ox = 0;
        if(-0.0005<oy&&oy<0)oy = 0;
        double r = sqrt((x1-ox)*(x1-ox)+(y1-oy)*(y1-oy));
        printf("%.3f %.3f %.3f\n",ox,oy,r);
        --n;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef double D;
typedef complex<D> P;
typedef vector<P> VP;
P minEnclosingCircle(const VP& ps) {
  P c;
  double move = 0.5;
  r(i,39) {  // 2^(-39-1) \approx 0.9e-12
    r(t,50) {
      D max = 0;
      int k = 0;
      r (j, ps.size()) if (max < norm(ps[j]-c)) {
        max = norm(ps[j]-c);
        k = j;
      }
      c += (ps[k]-c) * move;
    }
    move /= 2;
  }
  return c;
}
int main(){
    int n;
    cin>>n;
    r(i,n){
        VP p;
        D a,b;
        r(j,3){
            cin>>a>>b;
            p.push_back(P(a,b));
        }
        P t=minEnclosingCircle(p);
        D dis=abs(p[0]-t);
        printf("%.3f %.3f %.3f\n",t.real(),t.imag(),dis);
    }
}

C++: 
#include <iostream>
#include <cstdio>
#include <complex>
#include <cmath>

using namespace std;

typedef complex<double> Point;

bool eqv(double d1, double d2) {
    return abs(d1-d2) < 1e-10;
}

double r_(double x) {
    if(abs(x) < 0.001) return 0;
    return x;
}

int main() {
    int N;

    cin >> N;
    while(N--) {
        double x1, x2, x3, y1, y2, y3;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        Point p1(x1, y1), p2(x2, y2), p3(x3, y3);

        double R = abs(abs(p3-p2) / sin(arg(p2-p1) - arg(p3-p1))) / 2;
        Point pc1 = (p1+p2) / 2.;
        Point ph1 = (pc1-p1) * Point(0, 1);
        Point pc2 = (p1+p3) / 2.;
        Point ph2 = (pc2-p1) * Point(0, 1);

        //pc1 + k*ph1 == pc2 + l*ph2
        //pc1-pc2 == l*ph2 - k*ph1
        //d = pc1-pc2
        //l*ph2.x - k*ph1.x == d.x
        //l*ph2.y - k*ph1.y == d.y
        Point d = pc1-pc2;
        double det = ph2.real()*-ph1.imag() + ph1.real()*ph2.imag();
        double l = (d.real()*-ph1.imag() - d.imag()*ph2.imag()) / det;
        double k = (d.real()*ph1.real() + d.imag()*ph2.real()) / det;
        Point center = pc1 + ph1 * k;
        R = abs(center-p1);

        printf("%.3f %.3f %.3f\n", r_(center.real()), r_(center.imag()), r_(R));
    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main() {
  int n=0;
  double ax,ay,bx,by,cx,cy,S,R,a,b,c,x=0,y=0;
  cin>>n;
  for(int i=0; i<n; i++){
    cin>>ax>>ay>>bx>>by>>cx>>cy;
    S=((bx-ax)*(cy-ay)-(cx-ax)*(by-ay))/2;
    a=sqrt((ax-bx)*(ax-bx)+(ay-by)*(ay-by));
    b=sqrt((cx-bx)*(cx-bx)+(cy-by)*(cy-by));
    c=sqrt((ax-cx)*(ax-cx)+(ay-cy)*(ay-cy));
    cout<<a<<" "<<b<<" "<<c<<" "<<S<<endl;
    R=a*b*c/(4*S);
    x=(a*a*(b*b+c*c-a*a)*ax+b*b*(c*c+a*a-b*b)*bx+c*c*(a*a+b*b-c*c)*cx)/(16*S*S);
    y=(a*a*(b*b+c*c-a*a)*ay+b*b*(c*c+a*a-b*b)*by+c*c*(a*a+b*b-c*c)*cy)/(16*S*S);
    cout<<x<<" "<<y<<" "<<R<<endl;
  }
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main()
{
	int n;
	double x1, x2, x3;
	double y1, y2, y3;
	cin >> n;
	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
	double px, py, r;
	double A = (x2 - x1)*(y3 - y2) - (y2 - y1)*(x3 - x2);
	px = 0.5*((y3 - y2)*(x2*x2 - x1*x1 + y2*y2 - y1*y1) - (y2 - y1)*(x3*x3 - x2*x2 + y3*y3 - y2*y2)) / A;
	py = 0.5*(-(x3 - x2)*(x2*x2 - x1*x1 + y2*y2 - y1*y1) + (x2 - x1)*(x3*x3 - x2*x2 + y3*y3 - y2*y2)) / A;
	double a = px - x1;
	double b = py - y1;
	double r0 = (a*a + b*b);
	double r1 = sqrt(r0);
	cout << fixed << setprecision(10) << px << " " << py << " " << r1 <<  endl;
}