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++: /*
C++ ???????????????????????????????????¬??????
??????AOJ-ITP1??????????????????
*/
#define _USE_MATH_DEFINES

#include <iostream> // ?¨??????\??????
#include <fstream>  // ??????????????\??????
#include <string>   // ???????????¢???
#include <vector>   // ???????????????
#include <cmath>    // ??°??????
#include <iomanip>  // stream???????????¢???
#include <algorithm>// max??¢??°??????

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;        // ????´???°????????????????????????
using std::fixed;         // ???????????????????¨???????????????????????????????
using std::setprecision;  // ?????°????°???°?????\???????????°????????????????????¨????????\???
using std::pow;

int main() {
	int num;
	double x1, y1, x2, y2, x3, y3;
	

	cin >> num;
	for (size_t i = 0; i < num; i++)
	{
		double p, q, r;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		p = ((y1 - y3)*(pow(y1, 2.0) - pow(y2, 2.0) + pow(x1, 2.0) - pow(x2, 2.0)) - (y1 - y2)*(pow(y1, 2.0) - pow(y3, 2.0) + pow(x1, 2.0) - pow(x3, 2.0))) / (2*(y1 - y3)*(x1 - x2) - 2*(y1 - y2)*(x1 - x3));
		q = ((x1 - x3)*(pow(x1, 2.0) - pow(x2, 2.0) + pow(y1, 2.0) - pow(y2, 2.0)) - (x1 - x2)*(pow(x1, 2.0) - pow(x3, 2.0) + pow(y1, 2.0) - pow(y3, 2.0))) / (2*(x1 - x3)*(y1 - y2) - 2*(x1 - x2)*(y1 - y3));

		r = sqrt(pow((p - x1), 2.0) + pow((q - x2), 2.0));

		cout << fixed << setprecision(3) << p << " " << q  << " " << r << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <complex>
using namespace std;

#define x real()
#define y imag()
typedef complex<double> Point;

Point circumscribed_Circle_Center(Point a, Point b, Point c){
  double A = a.x - b.x;
  double B = a.y - b.y;
  double C = -1.0 * (a.x*a.x - b.x*b.x + a.y*a.y - b.y*b.y);
  double D = a.x - c.x;
  double E = a.y - c.y;
  double F = -1 * (a.x*a.x - c.x*c.x + a.y*a.y - c.y*c.y);

  double L = (C*E - B*F) / (E*A - B*D);
  double M = (D*C - A*F) / (D*B - E*A);
  double xp = L / -2.0;
  double yp = M / -2.0;
  return (Point(xp, yp));
}

double circumscribed_Circle_Radius(Point a, Point c){
  double L = c.x * -2.0;
  double M = c.y * -2.0;
  double N = -1 * (a.x*a.x + a.y*a.y + L*a.x + M*a.y);
  double r = sqrt(L*L + M*M - 4*N) / 2.0;
  return r;
}

int main(){
  int n;
  Point a, b, c;
  cin >> n;
  while(n--){
    cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;
    Point C = circumscribed_Circle_Center(a, b, c);
    double r = circumscribed_Circle_Radius(a, C);
    printf("%.3f %.3f %.3f\n", C.x, C.y, r);
  }
  return 0;
}

C++: #include <cstdio>
#include <vector>
#include <algorithm>
#include <cstring>
#include <string>
#include <cctype>
#include <sstream>
#include <cmath>
#include <climits>
#include <set>
#include <iostream>
#include <map>
#include <functional>
#include <cstdlib>
#include <numeric>
#include <queue>
#include <complex>
#include <sstream>
#include <stack>

using namespace std;

#define reep(i, f, n) for (int i = f; i < n; ++i)
#define rep(i, n) reep(i, 0, n)


int main(){
    float x1,x2,x3,y1,y2,y3,gx,gy,G,Xc,Yc,r;
    int n;
    scanf("%d", &n);
    for (int i=0;i<n;i++) {
        scanf("%f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3);
        gx = (x1+x2+x3)/3;
        gy = (y1+y2+y3)/3;
        G = ( y2*x1-y1*x2 +y3*x2-y2*x3 +y1*x3-y3*x1 );
        Xc = ((x1*x1+y1*y1)*(y2-y3)+(x2*x2+y2*y2)*(y3-y1)+(x3*x3+y3*y3)*(y1-y2))/(2*G);
        Yc = -((x1*x1+y1*y1)*(x2-x3)+(x2*x2+y2*y2)*(x3-x1)+(x3*x3+y3*y3)*(x1-x2))/(2*G);
        r = hypot(x1-Xc, y1-Yc);
        printf("%.3f %.3f %.3f\n", Xc, Yc, r);
    }
}