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++: //0010 Circumscribed Circle of A Triangle.
/*
½ÊãÌ_ (x1, y1)(x2, y2)(x3, y3) ð¸_Æµ½Op`Ì
OÚ~ÌSÀW(xp, yp)Æ¼a r ðoÍµÄI¹·évOðì¬µÄ­¾³¢B
x1, y1, x2, y2, x3, y3, xp, yp ÍA»ê¼ê -100 Èã 100 ÈºÌÀÆµÜ·B 

ÅÉf[^ZbgÌnª^¦çêÜ·
oÍÍA¬_Èºæ4ÅlÌÜüµÄAxp, yp, rð¼pXy[XÅæØÁÄ­¾³¢
*/

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


//¼a@http://blog.livedoor.jp/calc/archives/9248011.html
//OS_@http://homepage2.nifty.com/gis_yasu21/sub1.htm

template<class T>
struct Point
{
	T x, y;
	Point(){}
	Point(T x, T y):x(x),y(y){}
};
template<class T>
T Square(const T &x)
{
	return x*x;
}
template<class T>
T Distance(const Point<T> &a, const Point<T> &b)
{
	return sqrt( Square(a.x-b.x) + Square(a.y-b.y) );
}

int main(void)
{
	const int N = 3;
	int n;
	Point<double> p[N], o;
	double r;
	int i, j;

	cin >> n;
	for(i = 0; i < n; i++)
	{
		for(j = 0; j < N; j++)
			cin >> p[j].x >> p[j].y;

		//wÌö®ÅOÚ~ÌÊÏðßé
		double a, b, c, s, S;
		a = Distance(p[1], p[2]);
		b = Distance(p[2], p[0]);
		c = Distance(p[0], p[1]);
		s = (a+b+c)/2;
		S = sqrt(s*(s-a)*(s-b)*(s-c));
		//abc/4SªOS~Ì¼a@¿ÈÝÉS/sÍàS~Ì¼a
		r = (a*b*c) / (4*S);

		//S = (1/2)*bc*sinAæè
		double sinA, sinB, sinC;
		sinA = 2*S/b/c;
		sinB = 2*S/c/a;
		sinC = 2*S/a/b;
		//]·èæèAa^2 = b^2 + c^2 - 2bc*cosA
		double cosA, cosB, cosC;
		cosA = (Square(b)+Square(c)-Square(a)) / (2*b*c);
		cosB = (Square(c)+Square(a)-Square(b)) / (2*c*a);
		cosC = (Square(a)+Square(b)-Square(c)) / (2*a*b);
		//sin(2A) = 2*sinA*cosA
		double sin2A, sin2B, sin2C;
		sin2A = 2*sinA*cosA;
		sin2B = 2*sinB*cosB;
		sin2C = 2*sinC*cosC;
		//OS~ÌXÀWÍA(x1*sin2A + x2*sin2B + x3*sin2C) / (sin2A+sin2B+sin2C)AyÀWà¯l
		o.x = (p[0].x*sin2A + p[1].x*sin2B + p[2].x*sin2C) / (sin2A + sin2B + sin2C);
		o.y = (p[0].y*sin2A + p[1].y*sin2B + p[2].y*sin2C) / (sin2A + sin2B + sin2C);

		
		//¬_Èºà\¦·éæ¤ÉµA¬_æ4ðlÌÜü·é
		cout << setprecision(3);
		cout << setiosflags(ios::fixed);
		cout << o.x << ' ' << o.y << ' ' << r << endl;
	}

	return 0;
}

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

int main() {
    int n;
    double x1, y1, x2, y2, x3, y3;
    double px, py, r;
    
    cin >> n;
    while(n){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        px = ((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));
        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));
        r = sqrt((x1 - px) * (x1 - px) + (y1 - py) * (y1 - py));
        
        n--;

        printf("%.3f %.3f %.3f\n", px, py, r);
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
 
#define REP(i,s,n) for(int i=s;i<n;++i)
#define rep(i,n) REP(i,0,n)
#define SORT(c) sort((c).begin(),(c).end())
#define IINF INT_MAX
#define LLINF LLONG_MAX
 
typedef long long ll;
typedef pair<int, int> ii;
 
int main(){

	int n;
	cin >> n;
	rep(i, n){
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double vx21, vx31, vy21, vy31, tmp1, tmp2;
		vx21 = 2 * (x1 - x2);
		vx31 = 2 * (x1 - x3);
		vy21 = 2 * (y1 - y2);
		vy31 = 2 * (y1 - y3);
		tmp1 = x1 * x1 + y1 * y1 - x2 * x2 - y2 * y2;
		tmp2 = x1 * x1 + y1 * y1 - x3 * x3 - y3 * y3;
		
		double x, y, r;
		x = (vx31 * tmp1 - vx21 * tmp2) / (vx31 * vy21 - vx21 * vy31);
		y = (vy21 * tmp2 - vy31 * tmp1) / (vy21 * vx31 - vy31 * vx21);
		r = sqrt((x1 - x) * (x1 - x) + (y1 - y) * (y1 - y));
		
		printf("%.3lf %.3lf %.3lf\n", x, y, r);
		
	}
 
	return 0;
}