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 <bits/stdc++.h>
using namespace std;

#define iota(i,n,b,s) for(int i=int(b);i!=int((b)+(s)*(n));i+=(s))
#define range(i,n,m) iota(i,(((n)>(m))?((n)-(m)+1):((m)-(n)+1)),(n),((n)>(m)?-1:1))
#define rep(i,n) iota(i,(n),0,1)

#define INF (1e9)
#define EPS (1e-9)
#define cons(a,b) (make_pair(a,b))
#define car(a) (a.first)
#define cdr(a) (a.second)
#define cadr(a) (car(cdr(a)))
#define cddr(a) (cdr(cdr(a)))
#define all(a) a.begin(), a.end()
#define trace(var) cerr<<">>> "<<#var<<" = "<<var<<endl;

typedef long long Integer;
typedef double Real;
typedef pair<Real, Real> P; // Point
typedef pair<P, P> L; // segment or line
const Real PI = acos(-1);

template<class S, class T>
ostream& operator<<(ostream& os, pair<S,T> p) {
  os << '(' << car(p) << ", " << cdr(p) << ')';
  return os;
}

template<class T>
ostream& operator<<(ostream& os, vector<T> v) {
  os << v[0];
  for (int i=1, len=v.size(); i<len; ++i) os << ' ' << v[i];
  return os;
}

/* inner dot */
Real dot(P&a, P&b) {
  return car(a) * car(b) + cdr(a) * cdr(b);
}
Real operator*(P&a, P&b) {
  return car(a) * car(b) + cdr(a) * cdr(b);
}

/* scalar multiple */
P operator*(P a, Real c) {
  return cons(c * car(a), c * cdr(a));
}
P operator*(Real c, P a) {
  return cons(c * car(a), c * cdr(a));
}

P operator/(P a, Real d) {
  return cons(car(a) / d, cdr(a) / d);
}

Real det(P&a, P&b) {
  return car(a) * cdr(b) - cdr(a) * car(b);
}

/* vector operator */
P operator+(P a, P b) {
  return cons(car(a) + car(b), cdr(a) + cdr(b));
}

P operator-(P a) {
  return cons(-car(a), -cdr(a));
}

P operator-(P a, P b) {
  return cons(car(a) - car(b), cdr(a) - cdr(b));
}


Real Euclidean(P a, P b) {
  P p = a - b;
  return sqrt(pow(car(p), 2) + pow(cdr(p), 2));
}

/* 外接円 */
struct Circumcenter {
  P x; // 外心
  Real r; // 半径
  Circumcenter(P p0, P p1, P p2) {
    Real a = Euclidean(p1, p2);
    Real b = Euclidean(p2, p0);
    Real c = Euclidean(p0, p1);
    Real A = a * a;
    Real B = b * b;
    Real C = c * c;
    
    Real den = 2 * (A * B + B * C + C * A) - (A * A + B * B + C * C);
    Real dec_a = A * (B + C - A);
    Real dec_b = B * (C + A - B);
    Real dec_c = C * (A + B - C);

    x = (dec_a * p0 + dec_b * p1 + dec_c * p2) / den;

    r = Euclidean(x, p0);
  }
};

int main() {

  int n; cin >> n;

  while (n--) {
    Real x0, y0, x1, y1, x2, y2;
    cin >> x0 >> y0 >> x1 >> y1 >> x2 >> y2;
    Circumcenter c(P(x0,y0), P(x1,y1), P(x2,y2)); 
    printf("%0.3f %0.3f %0.3f\n", car(c.x), cdr(c.x),  c.r);
  }

  return 0;
}

C++: #include <algorithm>
#include <cmath>
#include <ctime>
#include <climits>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <vector>
#include <cstring>
#include <tuple>
#include <bitset>
#include <unordered_map>
using namespace std;
typedef long long ll;
int main() {
    int n;
    cin >> n;
    double x1, y1, x2, y2, x3, y3;
    for (int i=0; i<n; ++i) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        double a = (x1 - x2) / (y1 - y2) * (-1);
        double b = (y1 - y2) / 2 - (x1 - x2) / 2 * a;
        double c = (x2 - x3) / (y2 - y3) * (-1);
        double d = (y2 - y3) / 2 - (x2 - x3) / 2 * a;
        double px = (a - b) / (c - a);
        double py = (a * d - b * c) / (c - a);
        double r = hypot(x1 - px, y1 - py);
        cout.precision(3);
        cout << fixed << px << " " << py << " " << r << endl;
    }
    return 0;
}