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>

const static double	de_PI	= 3.14159265358979323846;
const static int	de_MOD	= 1000000007;
const static int	de_MAX	= 999999999;
const static int	de_MIN = -999999999;

inline double RoundOff(double a, const int n) {

	for (int i = 0; i < n - 1; i++) { a *= 10; }

	if (a > 0) { a += 0.5; }
	else { a -= 0.5; }

	a = static_cast<int>(a);

	for (int i = 0; i < n - 1; i++) { a /= 10; }

	return a;
}

inline double circumradius(std::complex<double> a, std::complex<double> b, std::complex<double> c) {

	//???
	double eAB = abs(b - a);
	double eBC = abs(c - b);
	double eCA = abs(a - c);

	//?§????(????????????)
	double aCAB = acos((eCA*eCA + eAB*eAB - eBC*eBC) / (2 * eCA*eAB));

	//?????\????????????(??£?????????)
	double R = 0.5*eBC / sin(aCAB);

	return(R);

}

inline std::complex<double> Circumcenter(std::complex<double> a, std::complex<double> b, std::complex<double> c) {

	//???
	double eAB = abs(b - a);
	double eBC = abs(c - b);
	double eCA = abs(a - c);

	//?§????(????????????)
	double aCAB = acos((eCA*eCA + eAB*eAB - eBC*eBC) / (2 * eCA*eAB));
	double aABC = acos((eAB*eAB + eBC*eBC - eCA*eCA) / (2 * eAB*eBC));
	double aBCA = acos((eBC*eBC + eCA*eCA - eAB*eAB) / (2 * eBC*eCA));

	double A = sin(2 * aCAB);
	double B = sin(2 * aABC);
	double C = sin(2 * aBCA);

	//???????????????????????????(?????????????????¢???????????¬???)
	std::complex<double> ans;
	ans.real((A*a.real() + B*b.real() + C*c.real()) / (A + B + C));
	ans.imag((A*a.imag() + B*b.imag() + C*c.imag()) / (A + B + C));

	return(ans);
}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	int n = 0;
	std::cin >> n;
	double x1 = 0, y1 = 0, x2 = 0, y2 = 0, x3 = 0, y3 = 0;

	std::complex<double> a, b, c, ans;
	double r = 0;
	for (int i = 0; i < n; i++) {
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a.real(x1);	a.imag(y1);
		b.real(x2);	b.imag(y2);
		c.real(x3);	c.imag(y3);
		ans = Circumcenter(a, b, c);
		r = circumradius(a, b, c);
		std::cout << std::fixed << std::setprecision(3) << RoundOff(ans.real(), 4);
		std::cout << " ";
		std::cout << std::fixed << std::setprecision(3) << RoundOff(ans.imag(), 4);
		std::cout << " ";
		std::cout << std::fixed << std::setprecision(3) << RoundOff(r, 4) << std::endl;
	}



}

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

typedef long long ll;
#define rep(i,n) for(int i=0;i<(n);++i)
#define dbg(x) cout << #x"="<< (x) << endl
#define fi first
#define se second
#define square(x) (x)*(x)

int main(){
	int n;
	cin >> n;
	vector<pair<double,double> > x(3);
	vector<double> d(3),r(3);
	pair<double,double> ans;
	double R;
	rep(j,n){
		rep(i,3){
			cin >> x[i].fi >> x[i].se;
		}
		d[0] = square(x[2].fi-x[1].fi) + square(x[2].se-x[1].se);
		d[1] = square(x[2].fi-x[0].fi) + square(x[2].se-x[0].se);
		d[2] = square(x[0].fi-x[1].fi) + square(x[0].se-x[1].se);
		r[0] = d[0]*(d[1]+d[2]-d[0]);
		r[1] = d[1]*(d[2]+d[0]-d[1]);
		r[2] = d[2]*(d[0]+d[1]-d[2]);
		R = r[0]+r[1]+r[2];
		ans.fi = (r[0]*x[0].fi+r[1]*x[1].fi+r[2]*x[2].fi)/R;
		ans.se = (r[0]*x[0].se+r[1]*x[1].se+r[2]*x[2].se)/R;
		R = square(x[0].fi-ans.fi) + square(x[0].se-ans.se);
		printf("%.3f %.3f %.3f\n",ans.fi,ans.se, sqrt(R));
	}
}