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 <cmath>

using namespace std;
typedef struct { double x; double y;} point;
 
point getCircumcenter(point a, point b, point c)
{
    double A1 = 2 * ( b.x - a.x );
    double B1 = 2 * ( b.y - a.y );
    double C1 = pow(a.x,2)-pow(b.x,2) + pow(a.y,2)-pow(b.y,2);
    double A2 = 2 * ( c.x - a.x );
    double B2 = 2 * ( c.y - a.y );
    double C2 = pow(a.x,2)-pow(c.x,2) + pow(a.y,2)-pow(c.y,2);
    double X = (B1 * C2 - B2 * C1) / (A1 * B2 - A2 * B1);
    double Y = (C1 * A2 - C2 * A1) / (A1 * B2 - A2 * B1);
	point P;
	P.x=X;
	P.y=Y;
    return P;
}

double getTwoPointDictance(point a, point b)
{
	return fabs(sqrt(pow((a.x-b.x),2)+pow((a.y-b.y),2)));
}
double AntiMinusZero(double n)
{
	if(n==0)
	{
		return 0;
	}
	return n;
}

int main()
{
    int n;
    cin>>n;
    point a,b,c;
    while(n--)
	{
        cin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
        point ans = getCircumcenter(a,b,c);
        printf("%.3lf %.3lf %.3lf", AntiMinusZero(ans.x), AntiMinusZero(ans.y), AntiMinusZero(getTwoPointDictance( a, ans)));
    }
}

C++: #include <iostream>
#include <cstdio>
#include <math.h>
using namespace std;
 
double heronsMethod(double a, double b, double c){
    double s = (a + b + c) / 2;
    return sqrt(s * (s - a) * (s - b) * (s - c));
}
 
int main()
{
    int n;
    cin >> n;
    for (int i = 0; i < n; i++){
        double x1, x2, x3, y1, y2, y3;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        double a = sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
        double b = sqrt((x2 - x3) * (x2 - x3) + (y2 - y3) * (y2 - y3));
        double c = sqrt((x3 - x1) * (x3 - x1) + (y3 - y1) * (y3 - y1));
        double s = heronsMethod(a, b, c);
        double x = (a*a * (b*b + c*c - a*a) *x3 + b*b * (c*c + a*a - b*b) *x1 + c*c *(a*a + b*b - c*c) *x2) / (16 * s * s);
        double y = (a*a * (b*b + c*c - a*a) *y3 + b*b * (c*c + a*a - b*b) *y1 + c*c *(a*a + b*b - c*c) *y2) / (16 * s * s);
        double r = (a * b * c) / sqrt((a + b + c) * (-a + b + c) * (a - b + c) * (a + b - c));
        printf("%.3f %.3f %.3f\n", x, y, r);
    }
}

C++: #include <iostream>
#include <vector>
#include <math.h>

using namespace std;

int main(){
  double x1,y1,x2,y2,x3,y3;
  vector<double> x1l,y1l,x2l,y2l,x3l,y3l;
  int n;

  cin >> n;

  for(int i=0; i<n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    x1l.push_back(x1);
    y1l.push_back(y1);
    x2l.push_back(x2);
    y2l.push_back(y2);
    x3l.push_back(x3);
    y3l.push_back(y3);
  }

  double ex,ey,fx,fy;
  double a,b,c,d,e,f,r;
  double cx,cy;

  for(int i=0; i<n; i++){
    x1=x1l[i];
    y1=y1l[i];
    x2=x2l[i];
    y2=y2l[i];
    x3=x3l[i];
    y3=y3l[i];

    a = x1*x1+y1+y1;
    b = x2*x2+y2+y2;
    c = x3*x3+y3+y3; 
    d = 0.5/(x1*y2-x2*y1+x2*y3-x3*y2+x3*y1-x1*y3);
    e = d*(a*y2-b*y1+b*y3-c*y2+c*y1-a*y3);
    f = d*(x1*b-x2*a+x2*c-x3*b+x3*a-x1*c);
    r = sqrt((e-x1)*(e-x1)+(f-y1)*(f-y1));
    printf("%0.3f %0.3f %0.3f\n",e,f,r);
  }

  r