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 <cmath>
#include <iomanip>

using namespace std;

struct point{
	double x;
	double y;
};

struct linear{
	double a;
	double b;
	double c;
};

linear get_linear(point a, point b){
	linear l;

	if (a.x == b.x){
		l.a = 1;
		l.b = 0;
		l.c = a.x;
	}
	else if (a.y == b.y){
		l.a = 0;
		l.b = 1;
		l.c = a.y;
	}
	else{
		l.a = (b.x - a.x) / (b.y - a.y);
		l.b = -1;
		l.c = a.x * l.a - a.y;
	}
	return l;
}

linear get_linear_vertical(point p, linear l){
	linear lv;
	lv.a = (-1)*l.b;
	lv.b = l.a;
	lv.c = lv.a*p.x + lv.b*p.y;
	return lv;
}

point get_cross_point(linear l1, linear l2){
	point p;
	p.x = (l1.b*l2.c - l1.c*l2.b) / (l1.b*l2.a - l1.a*l2.b);
	p.y = (l1.c*l2.a - l1.a*l2.c) / (l1.b*l2.a - l1.a*l2.b);
	return p;
}

double point_distance(point s, point e){
	double d;
	d = (e.x - s.x)*(e.x - s.x) + (e.y - s.y)*(e.y - s.y);
	d = sqrt(d);
	return d;
}

double round3dig(double d){
	d = d + (0.5 - (d < 0)) / 1000;
	d = int(d * 1000);
	if (d != 0) d /= 1000;
	return d;
}

int main()
{
	int n=1;
	double r;
	point p[3], mp, cp;
	linear l1, l2;

	cin >> n;
	vector<point> *v = new vector<point>[n];
	for (int i = 0; i < n; ++i){
		cin >> p[0].x >> p[0].y >> p[1].x >> p[1].y >> p[2].x >> p[2].y;
		v[i].push_back(p[0]);
		v[i].push_back(p[1]);
		v[i].push_back(p[2]);
	}
	for (int i = 0; i < n; ++i){
		l1 = get_linear(v[i].at(0), v[i].at(1));
		mp.x = (v[i].at(1).x + v[i].at(0).x) / 2;
		mp.y = (v[i].at(1).y + v[i].at(0).y) / 2;
		l1 = get_linear_vertical(mp, l1);

		l2 = get_linear(v[i].at(0), v[i].at(2));
		mp.x = (v[i].at(2).x + v[i].at(0).x) / 2;
		mp.y = (v[i].at(2).y + v[i].at(0).y) / 2;
		l2 = get_linear_vertical(mp, l2);

		cp = get_cross_point(l1, l2);
		r = point_distance(v[i].at(0), cp);

		cp.x = round3dig(cp.x);
		cp.y = round3dig(cp.y);
		r = round3dig(r);

		cout << setprecision(3);
		cout << setiosflags(ios::fixed);
		cout << cp.x << ' ' << cp.y << ' ' << r << endl;
	}
	return 0;
}

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

#define ROUND_RANK 4
#define EPS 1e-8

using namespace std;

struct vec2
{
	double x;
	double y;
};

//行列式の算出
double Calc_det(const double& p11, const double& p12, const double& p21, const double& p22) {
	return (p11*p22 - p12*p21);
}
//xの解の算出
double Calc_x(const double& det, const double& q1, const double& p22, const double& p12, const double& q2) {
	return ((q1*p22 - p12*q2) / det);
}
//yの解の算出
double Calc_y(const double& det, const double& p11, const double& q2, const double& q1, const double& p21) {
	return ((p11*q2 - q1*p21) / det);
}
//rの解の算出
double Calc_r(const double& x0, const double& y0, const double& x1, const double& y1) {
	double r;
	r = sqrt(pow(x1 - x0, 2) + pow(y1 - y0, 2));
	return r;
}
//少数点以下第rank位で四捨五入
double Round(const double& value, const int& rank) {
	int root = pow(10, rank);
	return (std::round(value*root) / root);
}

int main() {
	int n;
	vec2 ver[20][3];
	double p11, p12, p21, p22, q1, q2;
	double  det;//行列式
	double	x, y, r;//解

	cin >> n;

	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < 3; ++j) {
			cin >> ver[i][j].x;
			cin >> ver[i][j].y;
		}
		p11 = -2 * (ver[i][0].x - ver[i][1].x);
		p12 = -2 * (ver[i][0].y - ver[i][1].y);
		p21 = -2 * (ver[i][1].x - ver[i][2].x);
		p22 = -2 * (ver[i][1].y - ver[i][2].y);
		q1 = (ver[i][1].x + ver[i][0].x)*(ver[i][1].x - ver[i][0].x)
			+ (ver[i][1].y + ver[i][0].y)*(ver[i][1].y - ver[i][0].y);
		q2 = (ver[i][2].x + ver[i][1].x)*(ver[i][2].x - ver[i][1].x)
			+ (ver[i][2].y + ver[i][1].y)*(ver[i][2].y - ver[i][1].y);
		det = Calc_det(p11, p12, p21, p22);
		det = fabs(det);
		x = Calc_x(det, q1, p22, p12, q2);
		y = Calc_y(det, p11, q2, q1, p21);
		r = Calc_r(x, y, ver[i][0].x, ver[i][0].y);
		x = Round(x, ROUND_RANK);
		y = Round(y, ROUND_RANK);
		r = Round(r, ROUND_RANK);
		std::cout << std::fixed;
		std::cout << std::setprecision(3) << x + EPS << " " << y + EPS << " " << r + EPS << endl;
	}
}