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 <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <map>
#include <algorithm>

#define rep(i, j) FOR(i, 0, j)
#define FOR(i, j, k) for(int i = j; i < k; ++i)
#define SQ(i) ((i) * (i))
using namespace std;

int main(){
  double x1, y1, x2, y2, x3, y3;
  double a1, b1, c1, a2, b2, c2, x, y, a, b, c, cos_a, sin_a, r;
  int n;
  scanf("%d", &n);
  while(n--){
    scanf("%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3);
    a1 = 2 * (x2 - x1); a2 = 2 * (x3 - x1);
    b1 = 2 * (y2 - y1); b2 = 2 * (y3 - y1);
    c1 = SQ(x1) - SQ(x2) + SQ(y1) - SQ(y2);
    c2 = SQ(x1) - SQ(x3) + SQ(y1) - SQ(y3);
    x = (b1 * c1 - b2 * c1) / (a1 * b2 - a2 * b1);
    y = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);

    r = sqrt((x-x1) + (y - y1));
    printf("%.3f %.3f %.3f\n",x, y, r);
  }
  return 0;
}

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

int main()
{
    int num;
    cin >> num;
    double x1, x2, x3, y1, y2, y3;
    double px, py, r;

    for(int i = 0; i < num; ++i)
    {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        px = (1 / 2.0) * ((x1 * x1 + y1 * y1) * (y3 - y2) + (x2 * x2 + y2 * y2) * (y1 - y3)
            + (x3 * x3 + y3 * y3) * (y2 - y1)) / (x1 * (y3 - y1) + x2 * (y1 - y3) + x3 * (y2 - y1));
        py = -(x3 - x2) / (y3 - y2) * px + (1 / 2.0) * (x3 * x3 - x2 * x2 + y3 * y3 - y2 * y2) / (y3 - y2);
        r = sqrt((x1 - px) * (x1 - px) + (y1 - py) * (y1 - py));

        printf("%.3f %.3f %.3f\n", px, py, r);
        //cout << fixed << setprecision(3) << round(px * 1000) / 1000 << " " << round(py * 1000) / 1000 << " " << round(r * 1000) / 1000 << endl;
    }
}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;
typedef pair<double, double> PDD;
PDD calc(double mat[2][2], double vec[2]){
  double a = mat[0][0], b = mat[0][1], d = mat[1][0], e = mat[1][1];
  double c = vec[0], f = vec[1];
  double det = a * e - d * b;
  return make_pair((e * c - b * f) / det, (-c * d + a * f) / det);
}
int main(){
  int Tc;
  cin >> Tc;
  for(int tc = 0; tc < Tc; ++tc){
    double x1, y1, x2, y2, x3, y3;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double A, B, C, D, E, F;
    A = (y1 + y3) / 2.;
    B = (x1 - x3) / (y1 - y3);
    C = (x1 + x3) / 2.;
    D = (y2 + y3) / 2.;
    E = (x2 - x3) / (y2 - y3);
    F = (x2 + x3) / 2.;
    double mat[2][2];
    mat[0][0] = B; mat[0][1] = 1;
    mat[1][0] = E; mat[1][1] = 1;
    double vec[2];
    vec[0] = A + B * C;
    vec[1] = D + E * F;
    PDD res = calc(mat, vec);
    double x = res.first, y = res.second;
    double r = pow((x-x1)*(x-x1)+(y-y1)*(y-y1), 0.5);
    x = (double)(round(x*1000))/1000.0;
    y = (double)(round(y*1000))/1000.0;
    r = (double)(round(r*1000))/1000.0;
    x = (x == -0.0) ? 0.0 : x;
    y = (y == -0.0) ? 0.0 : y;
    printf("%.3f %.3f %.3f\n", x, y, r);
  }
  return 0;
}

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

using namespace std;

void square(double, double, double, double, double, double);
double edge(double, double, double, double);
void center(double, double, double, double, double, double);

double xp, yp;

int main () {

	double x1, y1, x2, y2, x3, y3, n;

	cin >> n;

	for(int i=0; i<n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		square(x1, y1, x2, y2, x3, y3);
	}

}


void square(double x1, double y1, double x2, double y2, double x3, double y3) {

	double r, a, b, c, s;

	a=edge(x1, x2, y1, y2);
	b=edge(x2, x3, y2, y3);
	c=edge(x3, x1, y3, y1);
	center(x1, y1, x2, y2, x3, y3);
	s=(a+b+c)/2;
	r=0.25*a*b*c/sqrt(s*(s-a)*(s-b)*(s-c));
	printf("%.3f %.3f %.3f\n", xp, yp, r);

}

double edge(double a, double b, double c, double d) {

	return sqrt(pow(a-b, 2)+pow(c-d, 2));

}

void center(double x1, double y1, double x2, double y2, double x3, double y3) {

	double a1=2*(x2-x1);
	double b1=2*(y2-y1);
	double c1=x1*x1-x2*x2+y1*y1-y2*y2;
	double a2=2*(x3-x1);
	double b2=2*(y3-y1);
	double c2=x1*x1-x3*x3+y1*y1-y3*y3;
	xp=(b1*c2-b2*c1)/(a1*b2-a2*b1);
	yp=(c1*a2-c2*a1)/(a1*b2-a2*b1);

}