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 <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>
#include <numeric>
#include <iomanip>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007
#define EPS 1e-10
#define MAX_N 20000
#define MAX_M 16

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<double, double> PD;
typedef pair<string, ll> PS;
typedef vector<ll> V;
typedef pair<P, char> PC;

int n;
double dx1, dx2, dx3, dy1, dy2, dy3;

int main(){
	cin >> n;
	REP(i, n){
		cin >> dx1 >> dy1 >> dx2 >> dy2 >> dx3 >> dy3;
		double a = 2 * (dx1 - dx2), b = 2 * (dy1 - dy2), c = (dx1*dx1 + dy1*dy1) - (dx2*dx2 + dy2*dy2);
		double d = 2 * (dx2 - dx3), e = 2 * (dy2 - dy3), f = (dx2*dx2 + dy2*dy2) - (dx3*dx3 + dy3*dy3);
		double x, y;
		if (a < 0){
			a = -a;
			b = -b;
			c = -c;
		}
		if (d < 0){
			d = -d;
			e = -e;
			f = -f;
		}
		if (a == 0){
			y = c / b;
			x = (f - e*y) / d;
		}
		else if (b == 0){
			x = c / a;
			if (d == 0) y = f / e;
			else y = (f - e*x) / d;
		}
		else if (a != 0 && b != 0){
			x = (b*f - c*e) / (b*d - a*e);
			y = (c*d - a*f) / (b*d - a*e);
		}
		cout << fixed << setprecision(3) << x << " " << y << " " << sqrt((dx1 - x)*(dx1 - x) + (dy1 - y)*(dy1 - y)) << endl;
	}
}

C++: #include <iostream>
#include <vector>
#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 round3dig(double d){
    d += 0.0005;
    d *= 1000;
    d = int(d);
    if(d != 0)
        d /= 1000;
    return d;
}

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

    cin >> n;
    vector< point > v[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;
        v[i].push_back(p[0]);
        v[i].push_back(p[1]);
        v[i].push_back(p[2]);
    }
    for(int i = 0; i < n; ++i){
        l1 = get_linear(v[i].at(0), v[i].at(1));
        mp.x = (v[i].at(1).x - v[i].at(0).x)/2;
        mp.y = (v[i].at(1).y - v[i].at(0).y)/2;
        l1 = get_linear_vertical(mp, l1);

        l2 = get_linear(v[i].at(0), v[i].at(2));
        mp.x = (v[i].at(2).x - v[i].at(0).x)/2;
        mp.y = (v[i].at(2).y - v[i].at(0).y)/2;
        l2 = get_linear_vertical(mp, l2);

        cp = get_cross_point(l1, l2);
        cp.x = round3dig(cp.x);
        cp.y = round3dig(cp.y);
        cout << fixed << setprecision(3)
             << cp.x << ' ' << cp.y << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;

int main(){

  long double x1,x2,x3,y1,y2,y3;
  long double p,q,r;
  int tmp;
  int n;

  cin >> n;

  while(n--){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

    p = ((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));

    q = ((x1-x3)*(x1*x1 -x2*x2 +y1*y1 -y2*y2) -(x1-x2)*(x1*x1 -x3*x3 +y1*y1 -y3*x3)) / (2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));

    r2 = sqrt((x1-p)*(x1-p) + (y1-q)*(y1-q));

    printf("%.3Lf %.3Lf %.3Lf\n", p, q, r);
  }
}