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<complex>
#include<vector>
#include<cmath>
#include<algorithm>
#include<cstdio>

using namespace std;

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

#define X real()
#define Y imag()

typedef long double D;
typedef complex<D> P;

// 内積
D dot(P a, P b) {return a.X * b.X + a.Y * b.Y;}
// 外積
D cross(P a, P b) {return a.X * b.Y - a.Y * b.X;}

D aTri(D a, D b, D c){
  D s = (a + b + c) / 2;
  return sqrt(max(s * (s - a) * (s - b) * (s - c), (D)0));
}

P circumscribedCircleCenter(P a, P b, P c) {
  P bc = c - b, ca = a - c, ab = b - a;
  return (b + c - bc * P(0, 1) * dot(ca, ab) / cross(ca, ab)) / (D)2;
}

D circumscribedCircleRadius(D a, D b, D c) {return a * b * c / (4 * aTri(a, b, c));}

int main() {
  int n;
  cin >> n;
  rep (i, n) {
    P a, b, c;
    cin >> a.X >> a.Y >> b.X >> b.Y >> c.X >> c.Y;
    P p = circumscribedCircleCenter(a, b, c);
    D r = circumscribedCircleRadius(abs(a - b), abs(b - c), abs(c - a));
    printf("%.3Lf %.3Lf %.3Lf\n", p.X, p.Y, r);
  }
  return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string>
#include <map>
#include <vector>
#include <set>
#include <iomanip>
#include <fstream>
#include <cstdint>
#include <cmath>
#include <algorithm>
#include <utility>
#include <numeric>
#include <functional>
using namespace std;

constexpr int INF       = 1000000000;/* 1e+9a */
constexpr int MODULO    = 1000000007;

#define int long

#define REP(i,f,n) for(int i=(f); i < (n); ++i)
#define PER(i,f,n) for(int i=(n-1); i >= f; --i)

constexpr int dx[4] = { 1, 0, -1, 0 };
constexpr int dy[4] = { 0, 1, 0, -1 };

//----------------------------------
//----------------------------------

template<typename STL> inline void print_STL(STL c, string ab = ""){
	cout << ab << c[0];
	for(int i = 1; i < c.size(); ++i) cout << ' ' << c[i]; }

template<typename STL> inline void import_STL(STL& c, int n){
	typename STL::value_type tmp;
	for(int i = 0; i < n; ++i){	cin >> tmp; c.push_back(tmp);} }

istream& operator>>(istream& os, pair<double,double>& x)
{
	double tmp;
	os >> tmp;
	x.first = tmp;
	os >> tmp;
	x.second = tmp;
	return os;
}

ostream& operator<<(ostream& os, pair<double,double>& x)
{
	return os << x.first << ' ' << x.second;
}

pair<double, double> operator-(pair<double,double> a, pair<double,double> b)
{
	return {a.first - b.first, a.second - b.second};
}
pair<double, double> operator+(pair<double,double> a, pair<double,double> b)
{
	return {a.first + b.first, a.second + b.second};
}

double operator*(pair<double, double> a, pair<double,double> b)
{
	return a.first * b.first + a.second * b.second;
}
double sq(double a)
{
	return a * a;
}

double round4(double x)
{
	x *= 1000;
	x = (int)(x + 0.5);
	return x / 1000;
}

signed main()
{
	pair<double,double> a, b, c;
	int n;
	cin >>n;
	while(cin >> a >> b >> c)
	{
		pair<double, double> va = b-c, vb = c-a;
		/* cout << va <<' ' << vb << '\n'; */
		auto m = b + c, n = c + a;
		m = {m.first / 2, m.second/2};
		n = {n.first / 2, n.second/2};
		double norm = va.first * vb.second - va.second * vb.first;
		double cx = va * m , cy = vb * n ;
		double sx = cx * vb.second - cy * va.second;
		double sy = va.first * cy - cx * vb.first;
		sx /= norm;
		sy /= norm;

		double r = sqrt(sq(c.first - sx)+ sq(c.second - sy));
		sx = round4(sx);
		sy = round4(sy);
		r = round4(r);
		printf("%.3lf %.3lf %.3lf\n", sx, sy, r);
	}
}

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

using namespace std;

#define MAX 256

float LOSER(float x){
	if((int)(x*10000)%10 > 4)return x + 0.001;
	return x;
}

int main(void){
	int n,i = 0;
	float x1,y1,x2,y2,x3,y3;
	float px[MAX],py[MAX],r[MAX];
	
	cin >> n;
	while(i < n){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		px[i] = ((y1-y3)*(y1*y1-y2*y2+x1*x1-x2*x2)+(y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3))/(2*(y1-y3)*(x1-x2)+2*(y1-y2)*(x1-x3));
		py[i] = ((x1-x3)*(x1*x1-x2*x2+y1*y1-y2*y2)+(x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3))/(2*(x1-x3)*(y1-y2)+2*(x1-x2)*(y1-y3));
		r[i] = sqrt((px[i]-x1)*(px[i]-x1)+(py[i]-y1)*(py[i]-y1));
		LOSER(px[i]);
		LOSER(py[i]);
		LOSER(r[i]);
		i++;
	}
	
	for(int i = 0;i < n;i++){
		scanf("%.3f %.3f %.3f",px[i],py[i],r[i]);
	}

	return 0;
}