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++: /* Karen {{{
  ___  ____
 |_  ||_  _|
   | |_/ /    ,--.   _ .--.  .---.  _ .--.
   |  __'.   `'_\ : [ `/'`\]/ /__\\[ `.-. |
  _| |  \ \_ // | |, | |    | \__., | | | |
 |____||____|\'-;__/[___]    '.__.'[___||__]
}}} */
// include {{{
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <cmath>
#include <string>
#include <vector>
#include <complex>
#include <iostream>
#include <algorithm>

using namespace std;

// }}}
/* cpp template {{{*/


#define endl '\n'
#define REP(i, n) for (int i = 0; i < (n); i++)
#define REPS(i, a, n) for (int i = (a); i < (n); i++)
#define RREP(i, n) for (int i = 1; i <= (n); i++)
#define X real()
#define Y imag()
#define EQ(n,m) (abs((n)-(m)) < EPS)

typedef double D;      // ??§?¨?????????????double???long double?????????
typedef long long ll;
typedef long double lb;
typedef complex<D> P;  // Point
typedef pair<P, P> L;  // Line
typedef vector<P> VP;
typedef vector<int> vi;
typedef vector<vector<int> > vii;
typedef pair<int, int> pii;
typedef pair<int, string> pis;
typedef pair<string, int> psi;

inline bool inside(int y, int x, int H, int W){return (y >= 0 && x >= 0 && y < H && x < W);}
inline int in() {int x; std::cin >> x; return x;}

template <typename T>
void print(std::vector<T>& v, char c = ' ') {
  REP(i, v.size()) {
    if (i != 0) std::cout << c;
    std::cout << v[i];
  }
  std::cout << endl;
}
template <typename T>
void print(T x) {
  std::cout << x << '\n';
}

/* ?????° */
const ll MOD = 1e9 + 7;
const ll INF = 1e9 + 9;
const D EPS  = 1e-9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1};
const int dy[] = {1, 0, -1, 0, 1, -1, -1, 1};


/* }}} */

D cross(P a, P b) {
  return (conj(a)*b).Y;
}

P crosspointLL(P a1, P a2, P b1, P b2) {
  D d1 = cross(b2-b1, b1-a1);
  D d2 = cross(b2-b1, a2-a1);
  if (EQ(d1, 0) && EQ(d2, 0)) return a1;  // same line
  if (EQ(d2, 0)) throw "kouten ga nai";   // ???????????????
  return a1 + d1/d2 * (a2-a1);
}

P circumcenter(P a, P b, P c) {
  a = (a-c)*0.5;
  b = (b-c)*0.5;
  return c + crosspointLL(a, a*P(1,1), b, b*P(1,1));
}

int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);

  int x = in();
  
  for (int i = 0; i < x; i++){
    D a1, a2, b1, b2, c1, c2;
    cin >> a1 >> a2 >> b1 >> b2 >> c1 >> c2;
    P p = circumcenter( P(a1, a2), P(b1, b2), P(c1, c2));
    D r = sqrt(pow(a1 - p.X, 2) + pow(a2 - p.Y, 2));
    printf("%.3f %.3f %.3f", p.X, p.Y, r);

    cout << endl;
  }
  return (0);
}

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

using namespace std;

struct point{
  double x, y;
};

struct circle{
  point p;
  double r;
};

circle circumscribed_circle(point p, point q, point r){
  double a[2],b[2],c[2];
  circle res;
    
    a[0]=2*(q.x-p.x);
    b[0]=2*(q.y-p.y);
    c[0]=p.x*p.x-q.x*q.x+p.y*p.y-q.y*q.y;
    
    a[1]=2*(r.x-p.x);
    b[1]=2*(r.y-p.y);
    c[1]=p.x*p.x-r.x*r.x+p.y*p.y-r.y*r.y;
    
    res.p.x=(b[0]*c[1]-b[1]*c[0])/(a[0]*b[1]-a[1]*b[0]);
    res.p.y=(c[0]*a[1]-c[1]*a[0])/(a[0]*b[1]-a[1]*b[0]);
 
    res.r=sqrt(pow(fabs(p.x-res.p.x),2)+pow(fabs(p.y-res.p.y),2));
    
    return res;
}

int main(void) {
  
  int n;
  point in[3];
  circle ans;
 
  cin >> n;
  
  while(n--){
    for(int i=0;i<3;i++)
      cin >> in[i].x >> in[i].y;

    ans=circumscribed_circle(in[0], in[1], in[2]);

    printf("%.3f %.3f %.3f\n",ans.p.x,ans.p.y,ans.r);
  }
  
  return 0;
}

C++: #include<iostream>
#include<cmath>
#include<algorithm>
#include<iomanip>
#include<cstdio>
using namespace std;
class Point {
public:
	double x, y;
};
double distance(Point x, Point y) {
	return pow(pow(fabs(x.x - y.x), 2) + pow(fabs(x.y - y.y), 2), 0.5);
}
int main() {
	cout << setprecision(4);
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		Point p[3], s;
		for (int i = 0; i < 3; i++) {
			cin >> p[i].x >> p[i].y;
		}
		double a, b, c, d, e, f;
		a = (-2)*p[0].x + 2 * p[1].x;
		b = (-2)*p[0].y + 2 * p[1].y;
		c = (-2)*p[0].x + 2 * p[2].x;
		d = (-2)*p[0].y + 2 * p[2].y;
		e = pow(p[0].x, 2) + pow(p[0].y, 2) - pow(p[1].x, 2) - pow(p[1].y, 2);
		f = pow(p[0].x, 2) + pow(p[0].y, 2) - pow(p[2].x, 2) - pow(p[2].y, 2);
		s.x = (b*f - d*e) / (a*d - b*c);
		s.y = (a*f - c*e) / (b*c - a*d);
		double k = distance(p[0], s);
		s.x = round(s.x * 1000) / 1000;
		s.y = round(s.y * 1000) / 1000;
		k = round(k * 1000) / 1000;
		printf("%.3f %.3f %.3f\n", s.x, s.y, k);
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
struct Point{double x,y;};
double dis(Point a,Point b){
  return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
Point tri(Point a, Point b, Point c){
  double a1,a2,b1,b2,c1,c2;
  a1=2*(b.x-a.x);
  b1=2*(b.y-a.y);
  c1=a.x*a.x-b.x*b.x+a.y*a.y-b.y*b.y;
  a2=2*(c.x-a.x);
  b2=2*(c.y-a.y);
  c2=a.x*a.x-c.x*c.x+a.y*a.y-c.y*c.y;
  Point p;
  p.x=(b1*c2-b2*c1)/(a1*b2-a2*b1);
  p.y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
  return p;
}
int main(){
  int n;
  cin >> n;
  int i;
  for(i=0;i<n;i++){
  Point a,b,c;
  cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;
  Point x = tri(a,b,c);
  printf("%.3f %.3f %.3f\n",x.x,x.y,dis(x,a));
  }
  return 0;
}

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

using namespace std;

int main(int argc, char **argv)
{
	int n;
	float x1, y1, x2, y2, x3, y3, a1, a2, b1, b2, c1, c2;
	cin >> n;
	while(n--){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a1 = 2*(x2-x1);
		b1 = 2*(y2-y1);
		c1 = x1*x1 - x2*x2 + y1*y1 - y2*y2;
		a2 = 2*(x3-x1);
		b2 = 2*(y3-y1);
		c2 = x1*x1 - x3*x3 + y1*y1 - y3*y3;
		
		x3 = (b1*c2-b2*c1)/(a1*b2-a2*b1);
		y3 = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		a1 = sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
		printf("%.3f %.3f %.3f\n", x3, y3, a1);
	}
	
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

double det(double a, double b, double c, double d, double e, double f, double g, double h, double i){
	double delta;
	double m[3][3] = {{a, b, c},{d, e, f},{g, h, i}};
	delta = m[0][0]*m[1][1]*m[2][2];
	delta += m[2][0]*m[0][1]*m[1][2];
	delta += m[0][2]*m[1][0]*m[2][1];
	delta -= m[0][2]*m[1][1]*m[2][0];
	delta -= m[0][0]*m[1][2]*m[2][1];
	delta -= m[2][2]*m[0][1]*m[1][0];
	return delta;
}

int main(){
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		double x1, y1, x2, y2, x3, y3, x, y, r;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double d0 = det(x1, y1, 1, x2, y2, 1, x3, y3, 1);
		double d1 = det((-((x1*x1)+(y1*y1))), y1, 1, (-((x2*x2)+(y2*y2))), y2, 1, (-((x3*x3)+(y3*y3))), y3, 1);
		double d2 = det(x1, (-((x1*x1)+(y1*y1))), 1, x2, (-((x2*x2)+(y2*y2))), 1, x3, (-((x3*x3)+(y3*y3))), 1);
		double d3 = det(x1, y1, (-((x1*x1)+(y1*y1))), x2, y2, (-((x2*x2)+(y2*y2))), x3, y3, (-((x3*x3)+(y3*y3))));
		double l, m, n;
		l = d1 / d0;
		m = d2 / d0;
		n = d3 / d0;
		x = -(l / 2);
		y = -(m / 2);
		r = sqrt((l*l + m*m - (4*n)) / 4);
		cout << setprecision(4) << x << " " << y << " " << r << endl;
	}
	return 0;
}