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

double det(double a, double b, double c, double d, double e, double f, double g, double h, double i){
	double delta;
	double m[3][3] = {{a, b, c},{d, e, f},{g, h, i}};
	delta = m[0][0]*m[1][1]*m[2][2];
	delta += m[2][0]*m[0][1]*m[1][2];
	delta += m[0][2]*m[1][0]*m[2][1];
	delta -= m[0][2]*m[1][1]*m[2][0];
	delta -= m[0][0]*m[1][2]*m[2][1];
	delta -= m[2][2]*m[0][1]*m[1][0];
	return delta;
}

int main(){
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		double x1, y1, x2, y2, x3, y3, x, y, r;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double d0 = det(x1, y1, 1, x2, y2, 1, x3, y3, 1);
		double d1 = det((-((x1*x1)+(y1*y1))), y1, 1, (-((x2*x2)+(y2*y2))), y2, 1, (-((x3*x3)+(y3*y3))), y3, 1);
		double d2 = det(x1, (-((x1*x1)+(y1*y1))), 1, x2, (-((x2*x2)+(y2*y2))), 1, x3, (-((x3*x3)+(y3*y3))), 1);
		double d3 = det(x1, y1, (-((x1*x1)+(y1*y1))), x2, y2, (-((x2*x2)+(y2*y2))), x3, y3, (-((x3*x3)+(y3*y3))));
		double l, m, n;
		l = d1 / d0;
		m = d2 / d0;
		n = d3 / d0;
		x = -(l / 2);
		y = -(m / 2);
		r = sqrt((l*l + m*m - (4*n)) / 4);
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <cassert>
#include <cmath>

using namespace std;

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

struct point {
	double x;
	double y;
};

struct circle {
	point center;
	double radius;
};

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

point get_midpoint(const point& p1, const point& p2)
{
	point mid;
	mid.x = p1.x + p2.x / 2.0;
	mid.y = p1.y + p2.y / 2.0;
	return mid;
}

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

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

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

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

	// c,p1,p2のなす角
	double theta = acos((a / 2.0) / result.radius);

	result.center.x = p1.x + result.radius * cos(theta);
	result.center.y = p1.y + result.radius * sin(theta);

	return result;
}

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

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;
}

C++:  /*
Write a program which prints the central coordinate (px, py) and the radius r of a circumscribed circle of a triangle which is constructed by three points (x1, y1)(x2, y2)(x3, y3) on the plane surface.
 */

#include <iostream>
#include <math.h>

using namespace std;

double sqr(double a, double b, double c, double d) {
	// ñÂÌ_Ì£ðßé\bh
	double s = sqrt(pow(max(a,c)-min(a,c), 2) + pow(max(b,d)-min(b,d), 2));
	return s;
}

void solve(double a, double b, double c, double d, double e, double f) {
	double px,py,r;
	// ¼ñªüñÂÌð_ y-y1 = y2-y1 / x2-x1 * x-x1 -> y2-y1=a, x2-x1=b, y = a/b*x - a/b*x1-y1 -> y = -b/a*x + c
	double sl1 = -1*(c-a)/(d-b);			// (x1, y1)-(x2, y2)Ì¼ñªüÌX«
	double sl2 = -1*(e-c)/(f-d);			// (x2, y2)-(x3, y3)Ì¼ñªüÌX«
	double mid1x = (a+c)/2; double mid1y = (b+d)/2;
	double mid2x = (c+e)/2; double mid2y = (d+f)/2;
	double t1 = mid1y-sl1*mid1x;
	double t2 = mid2y-sl2*mid2x;
	// y = sl1*x - t1 Æ y = sl2*x - t2 ðA§B -> sl1*x - t1 = sl2*x - t2 -> (sl1-sl2)*x = t1 - t2
	px = (t1-t2)/(sl1-sl2);
	py = sl1*px - t1;
	r = sqr(px,py,a,b);
	printf("%4.3f %4.3f %4.3f",px,py,r);
}

int main() {
	int n; cin >> n;
	double x1, y1, x2, y2, x3, y3;
	for (int i=0; i<n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		solve(x1,y1,x2,y2,x3,y3);
	}
	return 0;
}