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>
#define EPS 1e-10
using namespace std;

double simulx(double a, double b, double c, double d, double e, double f){
	return (double)(c*e - f*b) / (a*e - d*b) + EPS;
}

double simuly(double a, double b, double c, double d, double e, double f){
	return (double)(c*d - f*a) / (b*d - e*a) + EPS;
}

int main(void){
	int n;
	cin >> n;
	for(int i=0; i<n; i++){
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double px, py;
		double a, b, c, d, e, f;
		a = 2*(x1-x2); b = 2*(y1-y2); c=x1*x1-x2*x2+y1*y1-y2*y2;
		d = 2*(x1-x3); e = 2*(y1-y3); f=x1*x1-x3*x3+y1*y1-y3*y3;
		px = simulx(a, b, c, d, e, f);
		py = simuly(a, b, c, d, e, f);
		cout.setf(ios::fixed);
		cout.precision(3);
		cout << px << " " << py << " " << sqrt((x1-px)*(x1-px)+(y1-py)*(y1-py)) << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define LOG(...) fprintf(stderr, __VA_ARGS__)
//#define LOG(...)
#define FOR(i, a, b) for(int i=(int)(a); i<(int)(b); ++i)
#define REP(i, n) for(int i=0; i<(int)(n); ++i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EXIST(s, e) ((s).find(e)!=(s).end())
#define SORT(c) sort(ALL(c))
#define RSORT(c) sort(RALL(c))
#define SQ(n) (n) * (n)

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

int main() {
  int n;
  float x1, y1, x2, y2, x3, y3;
  cin >> n;
  REP(i, n){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    float a = 2 * (x2 - x1);
    float b = 2 * (y2 - y1);
    float e = x2 * x2 + y2 * y2 - x1 * x1 - y1 * y1;
    float c = 2 * (x3 - x2);
    float d = 2 * (y3 - y2);
    float f = x3 * x3 + y3 * y3 - x2 * x2 - y2 * y2;
    LOG("%f %f %f\n%f %f %f\n", a, b, e, c, d, f);

    float D = 1 / (a * d - b * c);
    float A = (d * e - b * f) * D;
    float B = (-c * e + a * f) * D;
    printf("%.3f %.3f %.3f\n", A, B, sqrt(SQ(x1-A)+SQ(x1-B)));
  }
}

C++: #include <iostream>
#include <complex>
#include <cmath>
#define X real()
#define Y imag()
#define F first
#define S second
#define ML make_pair
#define EPS 1e-10
#define PI acos(-1)
using namespace std;
typedef long double  D;
typedef complex<double> P;
typedef complex<double> V;
typedef pair<P,P> L;


// 内積 (Inner Product) : a・b = |a||b|cosθ 1
D IP(V a, V b) {
	return (a.real() * b.real() + a.imag() * b.imag());
}

// 外積 (Exterior Product) : |a×b| = |a||b|sinθ 2
D EP(V a, V b) {
	return (a.real() * b.imag() - a.imag() * b.real());
}

//交点判定
bool ISC(L l1,L l2){
	V v1,v2;
	v1=l1.S-l1.F;
	v2=l2.S-l2.F;
	if(abs(EP(v1,v2))<EPS)
		return false;
	else
		return true;
}

//　交点の座標
P IS(L l1,L l2){
	P p1,p2,p3,p4;
	p1=l1.F,p2=l1.S,p3=l2.F,p4=l2.S;
	D tmp=(EP(p4-p3,p3-p1)/EP(p4-p3,p2-p1));
	V v1=(tmp,tmp);
	return p1+(p2-p1)*v1;
}
int main(void){
	int n;
	cin >>n;
	for(int j=0;j<n;j++){
		P point[4],tmp1,tmp2;
		for(int i=0;i<3;i++)
			cin >> point[i].X >> point[i].Y;
		L l1,l2;
		tmp1=((point[0]+point[1])/P(2.0,0));
		D angle,r=1.0;
		angle=arg(point[1]-point[0]);
		angle+=PI/2.0;
		tmp2=polar(r,angle);
		l1=ML(tmp1,tmp1+tmp2);

		tmp1=(point[0]+point[2])/P(2.0,0);
		angle=arg(point[2]-point[0]);
		angle+=PI/2.0;
		tmp2=polar(r,angle);
		l2=ML(tmp1,tmp1+tmp2);

		point[3]=IS(l1,l2);
		cout.precision(3);
		cout << fixed << point[3].X << ' '<< point[3].Y << ' '<< abs(point[3]-point[0]) << endl;
	}
	return 0;
}