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 <vector>
//#include <math.h>
//#include <string>
#include <cmath>
#include <iomanip>

int main();
double round(double dSrc, int iLen);

int main(){
	int cnt = 0;		//データセットの個数
	double x1, y1, x2, y2, x3, y3;
	std::vector<double> p;		//外接円の中心のx座標
	std::vector<double> q;		//外接円の中心のy座標
	std::vector<double> r;		//外接円の半径
	
	std::cin >> cnt;
	for (int i = 0; i < cnt; i++){
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		p.push_back(((y1 - y3) * (pow(y1, 2) - pow(y2, 2) + pow(x1, 2) - pow(x2, 2)) - (y1 - y2) * (pow(y1, 2) - pow(y3, 2) + pow(x1, 2) - pow(x3, 2))) / (2 * (y1 - y3) * (x1 - x2) - 2 * (y1 - y2) * (x1 - x3)));
		q.push_back(((x1 - x3) * (pow(x1, 2) - pow(x2, 2) + pow(y1, 2) - pow(y2, 2)) - (x1 - x2) * (pow(x1, 2) - pow(x3, 2) + pow(y1, 2) - pow(y3, 2))) / (2 * (x1 - x3) * (y1 - y2) - 2 * (x1 - x2) * (y1 - y3)));
		r.push_back(sqrt((pow((p[i] - x1), 2) + pow((q[i] - y1), 2))));

		double result_p;
		double result_q;
		double result_r;

		result_p = round(p[i], 3);
		result_q = round(q[i], 3);
		result_r = round(r[i], 3);

		std::cout << std::fixed << std::setprecision(3) << result_p << " " << result_q << " " << result_r << std::endl;
	}
	/*
	for (int i = 0; i < cnt; i++){
		double result_p;
		double result_q;
		double result_r;

		result_p = round(p[i], 3);
		result_q = round(q[i], 3);
		result_r = round(r[i], 3);

		std::cout << std::fixed << std::setprecision(3) << result_p << " " << result_q << " " << result_r << std::endl;
	}
	*/

	//while (1);
	return 0;
}

/**
* 指定桁で四捨五入を行う
* args:数値, 四捨五入する桁数
* return:結果
* 参考:http://oresi.hatenablog.com/entry/2014/06/23/【C%2CC%2B%2B】指定桁数で端数処理%EF%BC%88切り上げ、切り下げ
*/
double round(double dSrc, int iLen)
{
	double	dRet;

	dRet = dSrc * pow(10.0, iLen);
	dRet = (double)(int)(dRet + 0.5);

	return dRet * pow(10.0, -iLen);
}

C++: #include <stdio.h>
#include <math.h>

int main()
{
  int setcount;
  float x1, y1, x2, y2, x3, y3;
  float nx1, ny1, nx2, ny2;
  float nr1, nr2;
  float xp, yp, r;
  int tx, ty, tr, rem;
  int i;

  scanf("%d", &setcount);
  for(i=0; i<setcount; i++){
    scanf("%f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3);
    nx1=x2-x1; ny1=y2-y1;
    nx2=x3-x1; ny2=y3-y1;
    nr1=nx1^2+ny1^2; nr2=nx2^2+ny2^2;
    nr1*=0.5f; nr2*=0.5f;

    if(nx1==0){
      nr1/=ny1; ny1=1.0f;
      nr2-=nr1*ny2; ny2=0.0f;
      nr2/=nx2; nx2=1.0f;
    }
    else {
      //normalize
      ny1/=nx1; nr1/=nx1; nx1=1.0f;
      //subtraction
      ny2-=ny1*nx2; nr2-=nr1*nx2; nx2=0.0f;
      //normalize
      nr2/=ny2; ny2=1.0f;
      //subtraction
      nr1-=nr2*ny1; ny1=0.0f;
    }


    printf("%.3f %.3f %.3f\n", xp, yp, r);
  }

  return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n; cin >> n;
    double x1, y1, x2, y2, x3, y3;
    for (int i=0; i<n; ++i) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        if (y1 == y3) swap(y3, y2);
        if (y2 == y3) swap(y3, y1);
        double px, py, r;
        double a = x2 - x3;
        double b = y2 - y3;
        double c = x2 + x3;
        double d = y2 + y3;
        double e = x3 - x1;
        double f = y3 - y1;
        double g = x3 + x1;
        double h = y3 + y1;
        px = ((a/b)*(c/2) + d/2 - (e/f)*(g/2) - h/2) / (a/b - e/f);
        py = -(a/b) * px + (a/b) * c/2 + d/2;
        r = sqrt((x1 - px) * (x1 - px) + (y1 - py) * (y1 - py));
        cout.precision(3);
        cout << fixed << px << " " << py << " " << r << "\n";
    }
    return 0;
}

C++: #include<complex>
#include<cstdio>
#include<cmath>
using namespace std;
typedef complex<double> P;
class Tr{
	P a,b,c;double A,B,C;double x,y,z;
	double F(double a,double b, double c){
		return acos((b*b+c*c-a*a)/(2*b*c));
	}
public:
	Tr(P p1, P p2, P p3){
		a=p1;b=p2;c=p3;B=abs(c-a);A=abs(b-c);C=abs(a-b);x=F(A,B,C);y=F(B,C,A);z=F(C,A,B);
	}
	double R(){
		return A/sin(x)/2.;
	}
	double X(){
		double A=sin(2*x),B=sin(2*y),C=sin(2*z);
		return(a.real()*A+b.real()*B+c.real()*C)/(A+B+C);
	}
	double Y(){
		double A=sin(2*x),B=sin(2*y),C=sin(2*z);
		return(a.imag()*A+b.imag()*B+c.imag()*C)/(A+B+C);
	}
};
int main(){
	int n,i;
	double x1,x2,x3,y1,y2,y3;
	scanf("%d",&n);
	for(i=0;i<n;i++){
		scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);
		P a(x1,y1),b(x2,y2),c(x3,y3);
		Tr T(a,b,c);
		printf("%.3f %.3f %.3f\n",T.X(),T.Y(),T.R());
	}
}