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;

void calculate(double, double, double, double, double, double, double&, double&);

int main(){
  int i, num;
  double x1, y1, x2, y2, x3, y3;
  double a, b, c, d, e, f;
  double l, m, n;
  double xp, yp, r;

  cin >> num;

  for(i=0; i<num; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

    a = x1 - x2;
    b = y1 - y2;
    c = x2*x2 + y2*y2 - (x1*x1 + y1*y1);
    d = x2 - x3;
    e = y2 - y3;
    f = x3*x3 + y3*y3 - (x2*x2 + y2*y2);

    calculate(a, b, c, d, e, f, l, m);
    n = -(x1*x1 + y1*y1 + l*x1 + m*y1);

    xp = -l / 2;
    yp = -m / 2;
    r = 0.5 * sqrt(l*l + m*m - 4*n);

    cout.setf(ios::fixed, ios::floatfield);
    cout.precision(3);
    cout << xp << " " << yp << " " << r << endl;
  }

  return 0;
}

void calculate(double a, double b, double c, double d, double e, double f, double& l, double& m){
    l = (c*e - f*b)/(a*e - d*b);

    if(b != 0)
      m = (c - a*l)/b;
    else
      m = (f - d*l)/e;
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>
using namespace std;
double ff(double a,int b){
  
  a*=10000.0;
  //  cout<<a<<endl;
  int c = a;
  if(c%10>4)c+=10;
  c/=10;
  return (double)c/1000.0;
}



double ax,ay,bx,by,cx,cy,ox,oy,dx,dy,ex,ey;
//ack はACの傾き  eos はEOの切片
double abk,ack,dok,eok,eos,dos;
int n;
int main(){

  cin>>n;
  for(int i=0;i<n;i++){
    cin>>ax>>ay>>bx>>by>>cx>>cy;
    //ABの中点をD,  ACの中点をE 外接円の中心をO とする
    dx=(ax+bx)/2.0;
    dy=(ay+by)/2.0;
    ex=(ax+cx)/2.0;
    ey=(ay+cy)/2.0;
    
    if(ax==bx){
      abk=(ay-by)/0.0000001;
    }else{
      abk=(ay-by)/(ax-bx);
    }
    if(abk==0.0)abk=0.0000001;
    dok=-1.0/abk;
    dos=dy-dx*dok;
    
    
    if(ax==cx){
      ack=(ay-cy)/0.0000001;
    }else{
      ack=(ay-cy)/(ax-cx);
    }
    if(ack==0.0)ack=0.0000001;
    eok=-1.0/ack;
    eos=ey-ex*eok;
    
    //Y  =  dok X  + dos
    //Y  =  eok X  + eos
    //dok X + dos = eok X + eos
    
    double Y,X;
    if(dok-eok==0.0){
      X=(eos-dos)/0.0000001;
    }else{
      X=(eos-dos)/(dok-eok);
    }
    Y= dok * X + dos;
    
    printf("%.3f %.3f ",ff(X,3),ff(Y,3));
    //cout<<ax<<' '<<X<<' '<<ay<<' '<<Y<<endl;
    printf("%.3f\n" ,  ff( sqrt( pow(ax-X,2) + pow(ay-Y,2) )   ,3)   );
    
    
  }
  return 0;
}

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;
}

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

	cin >> 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;

		l1 = get_linear(p[0], p[1]);
		mp.x = (p[1].x + p[0].x) / 2;
		mp.y = (p[1].y + p[0].y) / 2;
		l1 = get_linear_vertical(mp, l1);

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

		cp = get_cross_point(l1, l2);
		r = point_distance(p[0], cp);

		cout << setprecision(3);
		cout << setiosflags(ios::fixed);
		cout << round(cp.x*1000.0)/1000.0 << ' '
			 << round(cp.y*1000.0)/1000.0 << ' '
			 << round(r*1000.0)/1000.0 << endl;
	}
	return 0;
}