Go: package main

import (
	"fmt"
	"math"
)

func main() {
	n := scanInt()
	for i := 0; i < n; i++ {
		cal()
	}
}

func scanInt() int {
	var n int
	fmt.Scanf("%d", &n)
	return n
}

func cal() {
	var x1, y1, x2, y2, x3, y3 float64
	fmt.Scanf("%f%f%f%f%f%f", &x1, &y1, &x2, &y2, &x3, &y3)
	a := 2 * (x1 - x2)
	b := 2 * (y1 - y2)
	c := 2 * (x1 - x3)
	d := 2 * (y1 - y3)
	e := x1*x1 - x2*x2 + y1*y1 - y2*y2
	f := x1*x1 - x3*x3 + y1*y1 - y3*y3

	px := (d*e - b*f) / (a*d - b*c)
	py := (a*f - c*e) / (a*d - b*c)

	r := math.Sqrt((x1-px)*(x1-px) + (y1-py)*(y1-py))
	fmt.Printf("%.3f %.3f %.3f\n", px, py, r)
}

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

struct TCoordinate
{
	double x;
	double y;
};

TCoordinate GetCenterPoint(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	double x1 = P1.x;
	double x2 = P2.x;
	double x3 = P3.x;
	double y1 = P1.y;
	double y2 = P2.y;
	double y3 = P3.y;
	double x, y, r;
	if(y1 == y2 || y2 == y3 || y3 == y1){
		if(y2 == y3){
			swap(x3, x1);
			swap(y3, y1);
		}else if(y3 == y1){
			swap(x2, x3);
			swap(y2, y3);
		}
		double xa = (x2 + x3) / 2;
		double ya = (y2 + y3) / 2;
		double a  = (-x3 + x2) / (y3 - y2);
		x = (x1 + x2) / 2;
		y = a * (x - xa) + ya;
	}else{
		double xa = (x1 + x2) / 2;
		double ya = (y1 + y2) / 2;
		double a  = (-x2 + x1) / (y2 - y1);
		double xb = (x2 + x3) / 2;
		double yb = (y2 + y3) / 2;
		double b  = (-x3 + x2) / (y3 - y2);
		x = (b * xb - a * xa - yb + ya) / (b - a);
		y = a * (x - xa) + ya;
	}
	return {x, y};
}

double GetRadius(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	double x1 = P1.x;
	double x2 = P2.x;
	double x3 = P3.x;
	double y1 = P1.y;
	double y2 = P2.y;
	double y3 = P3.y;
	double x, y, r;
	if(y1 == y2 || y2 == y3 || y3 == y1){
		if(y2 == y3){
			swap(x3, x1);
			swap(y3, y1);
		}else if(y3 == y1){
			swap(x2, x3);
			swap(y2, y3);
		}
		double xa = (x2 + x3) / 2;
		double ya = (y2 + y3) / 2;
		double a  = (-x3 + x2) / (y3 - y2);
		x = (x1 + x2) / 2;
		y = a * (x - xa) + ya;
	}else{
		double xa = (x1 + x2) / 2;
		double ya = (y1 + y2) / 2;
		double a  = (-x2 + x1) / (y2 - y1);
		double xb = (x2 + x3) / 2;
		double yb = (y2 + y3) / 2;
		double b  = (-x3 + x2) / (y3 - y2);
		x = (b * xb - a * xa - yb + ya) / (b - a);
		y = a * (x - xa) + ya;
	}
	return sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1));
}

int main()
{
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		TCoordinate P1, P2, P3;
		cin >> P1.x >> P1.y >> P2.x >> P2.y >> P3.x >> P3.y;
		TCoordinate CenterPoint = GetCenterPoint(P1, P2, P3);
		double Radius = GetRadius(P1, P2, P3);
		cout << fixed << setprecision(3) << CenterPoint.x << ' ' << CenterPoint.y << ' ' << Radius << endl;
	}
	return 0;
}

C++: /*
NOTE :
*min_element(.begin(),.end())
*max_element(.begin(),.end())
priority_queue<> pq
 push()  pop() size()  top()
map<>
 find() begin() end() insert(map<>::value_type())
pair<>
sort(.begin(), .end())
sort(arr, arr+N)
sort(a, a+N, greater<int>())
binary_search(vc.begin(), vc.end(), x)  //binary_search???true, false?????????
vector<int>::iterator lit = lower_bound(vc.begin(), vc.end(), x);
  lower_bound?????????????????????"??\???"????????????????????????????????????????????¬???????????????
vector<int>::iterator uit = upper_bound(vc.begin(), vc.end(), x);
  upper_bound????????????????????????"?¶???????"?????????????????????????????????????????¬???????????????
*/
#include <iostream>
#include <cstdio>
#include <climits>
#include <cmath>
#include <cassert>
#include <algorithm>
#include <valarray>
#include <utility>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <map>
#include <set>

#define MAX 100
#define INF 1<<20
#define LLINF 10e12 // 1<<31 // 2^31
#define REP(i,n) for(int i=0;i<n;i++)
#define REP2(i,a,n) for(int i=(a);i<(n);i++)
#define SET(a,c) memset(a,c,sizeof a)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define FOREACH(i,c) for(auto (i) : (c))
#define BIT(i,j) ((i)>>(j))&1
#define ALL(o) (o).begin(), (o).end()
#define ERASE(o) (o).erase(unique(ALL((o))), (o).end());

using namespace std;
typedef long long ll;
typedef valarray<int> array_t;
/////
template <class T>
void printMartix(valarray<valarray<T> > &A)
{
  int n = A.size();
  int m = A[0].size();
  REP(i,n){{REP(j,n) cout << A[i][j] << " ";} cout << endl;}
  cout << endl;
}

template <class T>
void printArray(valarray<T> &A)
{
  int n = A.size();
  REP(i,n) cout << A[i] << " ";
  cout << endl << endl;
}

template <class T>
bool solution(valarray<valarray<T> > &A, valarray<T> &b)
{
  int n = A.size();
  REP(i,n)
  {
    T a;
    //printMartix(A);
    //printArray(b);
    REP2(u,i,n)
    {
      a = A[u][i];
      if(a == 0) continue;
      else
      {
        if(u != i)
        {
          swap(A[i],A[u]);
          swap(b[i],b[u]);
        }
        break;
      }
      if(u == n-1) return false;
    }
    A[i] /= (1.0 * a);
    b[i] /= (1.0 * a);
    REP(j,n)
    {
      if(j == i) continue;
      b[j] -= b[i] * A[j][i];
      A[j] -= A[i] * A[j][i];
    }
    //printMartix(A);
    //printArray(b);
  }
  return true;
}

/////
int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  // FROM HERE
  int n; cin >> n;
  REP(u,n)
  {
    double x[3], y[3]; REP(i,3) cin >> x[i] >> y[i];
    valarray<valarray<double> > A(valarray<double>(0.0,3),3);
    valarray<double> b(0.0,3);
    REP(i,3)
    {
      A[i][0] = x[i];
      A[i][1] = y[i];
      A[i][2] = 1;
      b[i] = -(x[i]*x[i] + y[i]*y[i]);
    }
    solution(A,b);
    double cx = -b[0] * 0.5;
    double cy = -b[1] * 0.5;
    double r = sqrt((x[0]-cx)*(x[0]-cx)+(y[0]-cy)*(y[0]-cy));
    printf("%.3lf %.3lf %.3lf\n",cx,cy,r);
  }
  return 0;
}

C++: #include <iostream>
#include <cmath>
#include <iomanip>
#define sqr(x) ((x) * (x))
using namespace std;

void equation(double a, double b, double c, double d, double e, double f, double *x, double *y)
{
  *x = (c * e - b * f) / (a * e - b * d);
  *y = (a * f - c * d) / (a * e - b * d);   
}


int main()
{
  int n;
  double x1, y1, x2, y2, x3, y3, x, y;
  cin >> n;
  for(int i = 0; i < n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    x = y = 0;
    equation((2 * (x2 - x1)), (2 * (y2 - y1)), (sqr(x2) + sqr(y2) - sqr(x1) - sqr(y1)), (2 * (x3 - x2)), (2 * (y3 - y2)), (sqr(x3) + sqr(y3) - sqr(x2) - sqr(y2)), &x, &y);
    cout << fixed << setprecision(3) << x << ' ' << y << ' ' << sqrt(sqr(x1 - x) + sqr(y1 - y)) << endl;
  }
}

C++: #include<cstdio>
#include<cmath>
using namespace std;
int main(){
	int n;
	scanf("%d",&n);
	while(n--){
		double a[6],matrix[4],ans[2];
		scanf("%lf %lf %lf %lf %lf %lf",&a[0],&a[1],&a[2],&a[3],&a[4],&a[5]);
		matrix[0] = 2*(a[0]-a[2]);
		matrix[1] = 2*(a[1]-a[3]);
		matrix[2] = 2*(a[0]-a[4]);
		matrix[3] = 2*(a[1]-a[5]);
		ans[0] = a[0]*a[0]+a[1]*a[1]-a[2]*a[2]-a[3]*a[3];
		ans[1] = a[0]*a[0]+a[1]*a[1]-a[4]*a[4]-a[5]*a[5];
		double t = matrix[0]*matrix[3]-matrix[1]*matrix[2];
		double x = matrix[3]*ans[0]/t-matrix[1]*ans[1]/t;
		double y = matrix[0]*ans[1]/t-matrix[2]*ans[0]/t;
		printf("%.3lf %.3lf %.3lf\n",x,y,sqrt((a[0]-x)*(a[0]-x)+(a[1]-y)*(a[1]-y)));
	}
}