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<iomanip>
    #include<cmath>
    using namespace std;

    int main ()
    {
        int num;
        double x1,y1,x2,y2,x3,y3;

        cin >> num;

        for( int i = 0; i < num; i++ )
        {
            double px = 0, py = 0, r = 0;
            cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

            px = ((x1*x1-x3*x3+y1*y1-y3*y3)*(y2-y1)-(x1*x1-x2*x2+y1*y1-y2*y2)*(y3-y1)) / (2*(x2-x1)*(y3-y1)-2*(x3-x1)*(y2-y1));
            py = ((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));
            //py = ((2*(x1-x2)*px+x2*x2-x1*x1+y2*y2-y1*y1)) / (2*(y2-y1));
            r = sqrt((px-x1)*(px-x1) + (py-y1)*(py-y1));

            cout << fixed;
            cout << setprecision(3) << px << " " << py << " " << r << endl;
        }

        return 0;
    }

C++: #include <iostream>
#include <algorithm>
#include <cassert>
#include <cmath>
#include <stdio.h>

using namespace std;

//-------------------------------------------------------------------------
// geometric library

namespace geometry {

struct point {
	double x;
	double y;
};

struct circle {
	point center;
	double radius;
};

struct line { // ax + by + c = 0
	double a;
	double b;
	double c;
};

std::istream& operator >> (std::istream& is, point& p)
{
	is >> p.x >> p.y;
	return is;
}

bool operator == (const point& p1, const point& p2)
{
	return p1.x == p2.x && p1.y == p2.y;
}

///! 2点の距離
double get_distance(const point& p1, const point& p2)
{
	return sqrt(pow(p1.x - p2.x, 2) + pow(p1.y - p2.y, 2));
}

///! 2直線の交点
bool get_crosspoint(const line& l1, const line& l2, point *result)
{
	double denominator = l1.a * l2.b - l1.b * l2.a;
	if (denominator == 0) {
		return false;
	}
	result->x = (l1.b * l2.c - l2.b * l1.c) / denominator;
	result->y = (l2.a * l1.c - l1.a * l2.c) / denominator;
	return true;
}

///! 2点の垂直2等分線
bool get_bisector(const point& p1, const point& p2, line *result)
{
	if (p1 == p2) {
		return false;
	}
	result->a = 2 * (p1.x - p2.x);
	result->b = 2 * (p1.y - p2.y);
	result->c = -(p1.y - p2.y) * (p1.y + p2.y) - (p1.x - p2.x) * (p1.x + p2.x);
	return true;
}

///! 外接円を求める
bool get_circum_circle(const point& p1, const point& p2, const point& p3, circle *result)
{
	double a = get_distance(p1, p2);
	double b = get_distance(p2, p3);
	double c = get_distance(p3, p1);

	// 半径の公式
	result->radius = a * b * c / sqrt((a + b + c) * (-a + b + c) * (a - b + c) * (a + b - c));

	line line_p1p2;
	line line_p2p3;

	// 頂点の垂直2等分線の交点が外心
	return get_bisector(p1, p2, &line_p1p2) &&
		   get_bisector(p2, p3, &line_p2p3) &&
		   get_crosspoint(line_p1p2, line_p2p3, &result->center);
}

} // namespace geometry

//-------------------------------------------------------------------------
// solution

using namespace geometry;

circle get_circum_circle(istream& is)
{
	point p1, p2, p3;
	circle result;
	is >> p1 >> p2 >> p3;

	if (get_circum_circle(p1, p2, p3, &result)) {
		return result;
	} else {
		assert(!"invalid input");
	}
}

int main(void)
{
	int num_cases;

	std::cin >> num_cases;

	for (int i = 0; i < num_cases; i++) {		
		circle result = get_circum_circle(std::cin);
		printf("%.3f %.3f %.3f\n", result.center.x, result.center.y, result.radius);
	}
	return 0;
}