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 <stdio.h>
#include <string>
#include <queue>
#include <cmath>

#define int64 long long int

using namespace std;

int main()
{
	int n;
	double x1,y1,x2,y2,x3,y3,x,y,r;
	double det,a1,b1,a2,b2,c1,c2;
	cin >> n;
	for(int i=0; i<n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a1 = 2 * (x1 - x2), b1 = 2 * (y1 - y2);
		a2 = 2 * (x2 - x3), b2 = 2 * (y2 - y3);
		det = a1 * b2 - a2 * b1;
		c1 = (x1*x1 - x2*x2) + (y1*y1 - y2*y2);
		c2 = (x2*x2 - x3*x3) + (y2*y2 - y3*y3);
		x = (a1 * c1 + b1 * c2) / det;
		y = (a2 * c1 + b2 * c2) / det;
		r = sqrt((x - x1)*(x - x1) + (y - y1)*(y - y1));
		if(fabs(x) < 1e-12) x = 0.0;
		if(fabs(y) < 1e-12) y = 0.0;
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}

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

using namespace std;

void solve()
{
    int n;
    cin >> n;
    while(n--)
	{
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double a1 = 2 * (x2 - x1);
		double a2 = 2 * (x3 - x1);
		double b1 = 2 * (y2 - y1);
		double b2 = 2 * (y3 - y1);
		double c1 = x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2;
		double c2 = x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3;
		double x = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
		double y = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
		double r = sqrt((x1 - x) * (x1 - x) + (y1 - y) * (y1 - y));
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
}

int main()
{
	solve();
	return(0);
}

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 = 2.0f * ( b.x - a.x );
    double B1 = 2.0f * ( b.y - a.y );
    double C1 = pow(a.x,2)-pow(b.x,2) + pow(a.y,2)-pow(b.y,2);
    double A2 = 2.0f * ( c.x - a.x );
    double B2 = 2.0f * ( 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 0.0f;
    }
    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 %.3lfn", AntiMinusZero(ans.x), AntiMinusZero(ans.y), getTwoPointDictance( a, ans));
    }
}

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

int main(){
	int n;
	double x1,y1,x2,y2,x3,y3,x1_2,x1_3,y1_2,y1_3,z1_2,z1_3,a,b,c,r;
	while(cin >> n){
		for(int i = 0;i < n;++i){
			cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
			z1_2 = (((x1 * x1) + (y1 * y1)) * -1) - (((x2 * x2) + (y2 * y2)) * -1);
			z1_3 = (((x1 * x1) + (y1 * y1)) * -1) - (((x3 * x3) + (y3 * y3)) * -1);
			x1_2 = x1 - x2;
			x1_3 = x1 - x3;
			y1_2 = y1 - y2;
			y1_3 = y1 - y3;
			a = ((y1_2 * z1_3) - (y1_3 * z1_2)) / ((y1_2 * x1_3) - (y1_3 * x1_2));
			b = ((x1_2 * z1_3) - (x1_3 * z1_2)) / ((x1_2 * y1_3) - (x1_3 * y1_2));
			c = ((x1 * x1) + (y1 * y1) + (a * x1) + (b * y1)) * -1;
			r = sqrt(c * 4 + (a * a) + (b * b));
			printf("%.3f %.3f %.3f\n",a / -2.0,b / -2.0,r);
		}
	}
	return 0;
}