Go: package main

import (
	"bufio"
	"fmt"
	"io"
	"math"
	"os"
	"strconv"
	"strings"

)


func main() {
	solve(os.Stdin, os.Stdout)
}

type ReaderEx struct {
	baseScanner *bufio.Scanner
	words       []string
	nextWordIdx func() int
	initWordIdx func(int)
}

func NewScannerEx(stdin io.Reader) *ReaderEx {
	var this ReaderEx
	var idx int
	var wordsCnt int
	this.baseScanner = bufio.NewScanner(stdin)
	this.baseScanner.Buffer(make([]byte, 0), 1E+11)
	this.nextWordIdx = func() int {
		cur := idx
		idx = (idx + 1) % wordsCnt
		return cur
	}
	this.initWordIdx = func(wc int) {
		idx = 0
		wordsCnt = wc
	}
	return &this
}

func (me *ReaderEx) Scan(doSplit bool) (isScanned bool) {
	isScanned = me.baseScanner.Scan()
	if doSplit {
		me.words = strings.Fields(me.baseScanner.Text())
	} else {
		me.words = []string{me.baseScanner.Text()}
	}
	me.initWordIdx(len(me.words))
	return
}

func (me *ReaderEx) nextBytes() []byte {
	return []byte(me.words[me.nextWordIdx()])
}
func (me *ReaderEx) nextString() string {
	return me.words[me.nextWordIdx()]
}

type StringArray []string

func (me *ReaderEx) nextStringArray() StringArray {
	return me.words[me.nextWordIdx():]
}

func (me StringArray) toStrings() []string {
	return me
}

func (me StringArray) toInts() []int {
	strValues := me.toStrings()
	values := make([]int, len(strValues))
	for i, v := range strValues {
		n, err := strconv.Atoi(v)
		if err != nil {
			panic(err)
		}
		values[i] = n
	}
	return values
}
func (me *ReaderEx) nextInt() int {
	n, err := strconv.Atoi(me.words[me.nextWordIdx()])

	defer func() {
		if err != nil {
			panic(fmt.Sprintf("getInt(%d)", n))
		}
	}()
	return n
}

func pow(x, y, mod int) (ans int) {

	ans = 1
	for bit := 1 << uint(math.Floor(math.Log2(float64(y)))); ; bit >>= 1 {
		if y&bit != 0 {
			ans *= x
			if mod > 0 {
				ans %= mod
			}
		}
		if bit <= 1 {
			return
		}
		ans *= ans
		if mod > 0 {
			ans %= mod
		}
	}
}

type factType struct {
	prime int
	count int
}

func fact(n int) (facts []*factType) {

	for i := 2; i*i <= n; i++ {
		if n%i == 0 {
			fa := factType{prime: i}
			for n%fa.prime == 0 {
				n /= fa.prime
				fa.count++
			}
			facts = append(facts, &fa)
		}
	}
	if n > 1 {
		facts = append(facts, &factType{prime: n, count: 1})
	}
	return facts
}
func dec2bin(n int) (ret []bool) {
	for bit := 1 << uint(math.Floor(math.Log2(float64(n)))); ; bit >>= 1 {
		ret = append(ret, n&bit != 0)

		if bit <= 1 {
			return
		}
	}

}

func abs(v int) int {
	if v < 0 {
		v = -v
	}
	return v
}

func b2i(b bool) (i int) {
	if b {
		i = 1
	} else {
		i = 0
	}
	return
}

func solve(stdin io.Reader, stdout io.Writer) {

	r := NewScannerEx(stdin)
	r.Scan(true)
	n := r.nextInt()

	type Vec struct {
		x, y float64
	}
	vec := func(a, b Vec) Vec { return Vec{b.x - a.x, b.y - a.y} }
	unv := func(a Vec) Vec { return Vec{-a.y, a.x} }
	cdot := func(a, b Vec) float64 { return a.x*b.x + a.y*b.y }
	times := func(t float64, a Vec) Vec { return Vec{t * a.x, t * a.y} }
	add := func(a, b Vec) Vec { return Vec{a.x + b.x, a.y + b.y} }
	for i := 0; i < n; i++ {

		r.Scan(true)
		nextFloat64 := func() (ret float64) { fmt.Sscanf(r.nextString(), "%f", &ret); return }
		a1, a2, a3 := Vec{nextFloat64(), nextFloat64()}, Vec{nextFloat64(), nextFloat64()}, Vec{nextFloat64(), nextFloat64()}
		unv23 := unv(vec(a2, a3))
		v12 := vec(a1, a2)
		v31 := vec(a3, a1)
		s := cdot(v12, v31) / 2 / cdot(v12, unv23)
		c := add(times(.5, add(a2, a3)), times(s, unv23))
		v1c := vec(a1, c)
		fmt.Fprintf(stdout, "%.3f %.3f %.3f\n", c.x, c.y, math.Sqrt(v1c.x*v1c.x+v1c.y*v1c.y))
	}
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>
#include <memory.h>
#include <map>
#include <math.h>
#include <utility>


#pragma warning(disable:4996)
using namespace std;
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;
const double PI = 3.141592653589793238;
const Int INF = 999999999999;
typedef pair<int, int> P;


double A[2][2];
double C[2];


int main()
{
	int n;
	cin >> n;
	REP(i, n){
		double x1, x2, x3, y1, y2, y3;
		cin >> x1>> x2 >> x3 >> y1 >> y2 >> y3;

		double x21 = x2 - x1;
		double x31 = x3 - x1;
		double y21 = y2 - y1;
		double y31 = y3 - y1;

		double xc = ((y31*(x21*x21 + y21*y21)) - (y21*(x31*x31 + y31*y31))) /
			(2 * (x21*y31 - y21*x31));

		double yc = ((x21*(x31*x31 + y31*y31)) - (x31*(x21*x21 + y21*y21))) /
			(2 * (x21*y31 - y21*x31));	
		
		double d = (xc - x1)*(xc - x1) + (yc - y1)*(yc - y1);
		int xc_ = xc * 1000;
		int yc_ = yc * 1000;
		xc_ += (xc_ % 10) > 4 ? 10 : -xc_%10;
		yc_ += (yc_ % 10) > 4 ? 10 : -yc_%10;

		printf("%.3lf %.3lf %.3lf\n", xc, yc, sqrt(d));


	}

}

C++: //ツ三ツ角ツ形ツづ個外ツ静堋円ツづ個陳?心ツづ? ツ三ツ角ツ形ツづ個静シツ閉ェツづ個青つ陳シツ禿アツ督卍閉ェツ静シツつェツ古ーツづュツづゥツ湘ェツ渉環づ可づ按づゥ.
//ツつアツづ個づヲツつ、ツづ按甘エツ可スツ学ツ的ツづ按姪「ツ妥ィツづーツ嘉ーツつュツづ?つォツづ? ツ暗按可コツづ個ペ-ツジツづ個δ可イツブツδ可δ環づーツ参ツ考ツづ可つキツづゥツづ?嘉ーツつォツづ「ツつキツつ「.
// - http://www.deqnotes.net/acmicpc/2d_geometry/
// - http://www.prefield.com/algorithm/index.html

#include <iostream>
#include <complex>
#include <vector>
#include <cstdio>
using namespace std;

typedef complex<double> P;

P p[3]; //ツ三ツ角ツ形ツづ個三ツ点

//ツ外ツ静?
double cross(const P &a, const P &b){
  return imag(conj(a) * b);
}

//ツ静シツ閉ェ/ツ陳シツ静シツ用ツづ個型
struct L : public vector<P> {
  L(const P &a, const P &b){
    push_back(a);
    push_back(b);
  }
};

//ツ陳シツ静シ2ツづつづ個古ーツ点ツづーツ仰?づ淞づゥ
P crosspoint(const L &l, const L &m){
  double A = cross(l[1] - l[0], m[1] - m[0]);
  double B = cross(l[1] - l[0], l[1] - m[0]);
  return m[0] + B / A * (m[1] - m[0]);
}

//ツ静シツ閉ェabツづ個青つ陳シツ禿アツ督卍閉ェツ静シツづーツ仰?づ淞づゥ
L getBisection(const P &a, const P &b){
  P s = (a + b) / 2.0; //ツ陳?点
  P t = (b - a) * P(0, 1); //ツ静シツ閉ェabツづ個法ツ静シツベツクツトツδ仰づーツ偲ヲツ督セ

  //ツ静シツ閉ェabツづ個青つ陳シツ禿アツ督卍閉ェツ静シツづ? ツ点sツつゥツづァs+tツづ鳴伸ツづ堕づゥツ陳シツ静シ
  return L(s, s + t);
}

void solve(){
  //ツ三ツ角ツ形ツづ個静シツ閉ェツづ個青つ陳シツ禿アツ督卍閉ェツ静シツづー2ツづつ偲ヲツ督セツつキツづゥ
  L l1 = getBisection(p[0], p[1]);
  L l2 = getBisection(p[1], p[2]);

  //ツ外ツ静堋円ツづ個陳?心ツづ? l1ツづ?2ツつェツ古ーツ債キツつオツつスツ点
  P o = crosspoint(l1, l2);

  //ツ三ツ角ツ形ツづ個任ツ暗督づ個陳クツ点ツつゥツづァ, ツ陳?心ツづ慊づ?づ個仰猟猟」ツづーツ偲ヲツづェツづ? ツ板シツ径ツづ可づ按づゥ
  printf("%.3f %.3f %.3f\n", o.real(), o.imag(), abs(o - p[0]));
}

int main(){
  int n;
  cin >> n;

  for(int i = 0; i < n; i++){
    for(int j = 0; j < 3; j++){
      cin >> p[j].real() >> p[j].imag();
    }

    solve();
  }
}

C++: #include<iostream>
#include<iomanip>
#include<memory>
#include<array>
#include<vector>
#include<queue>
#include<list>
#include<stack>
#include<algorithm>
#include<cmath>

using namespace std;

int main()
{
  int n;
  array<double, 3> x;
  array<double, 3> y;
  array<double, 2> a;
  array<double, 2> b;
  array<double, 2> c;
  double cx, cy, r;

  cin >> n;
  for ( int j = 0; j < n; j++ )
    {
      for ( auto i = 0; i < x.size(); i++ )
        {
          cin >> x[i];
          cin >> y[i];
        }
      
      for ( auto i = 0; i < a.size(); i++ )
        {
          a[i] =  y[i+1] - y[i];
          b[i] =  x[i+1] - x[i];
          c[i] = (x[i+1] * x[i+1] - x[i] * x[i]) * 0.5 + (y[i+1] * y[i+1] - y[i] * y[i]) * 0.5;
        }
      
      cx = (b[1] * c[0] - b[0] * c[1]) / (a[0] * b[1] - a[1] * b[0]);
      cy = (a[0] * c[1] - a[1] * c[0]) / (a[0] * b[1] - a[1] * b[0]);
      r = sqrt( (x[0] - cx) * (x[0] - cx) + (y[0] - cy) * (y[0] - cy) );
      
      cout << fixed << setprecision(3) << cx << " " << cy << " " << r << endl;
    }
  return 0;
}