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 <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <climits>
#include <cfloat>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <bitset>
#include <algorithm>
#include <functional>
#include <utility>
#include <numeric>

using namespace std;

typedef unsigned int UI;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int,int> PII;
typedef istringstream ISS;
typedef ostringstream OSS;

#define REP( i, m, n ) for ( int i = (int)( m ); i < (int)( n ); ++i )
#define EACH( v, c ) for ( auto &v : c )

#define ITER( c ) __typeof( (c).begin() )
#define IREP( c, it ) for ( ITER(c) it = c.begin(); it != c.end(); ++it )
#define ALL( c ) (c).begin(), (c).end()

#define PB( n ) push_back( n )
#define MP( a, b ) make_pair( ( a ), ( b ) )
#define EXIST( c, e ) ( (c).find( e ) != (c).end() )

#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl
#define DEBUG( x ) cerr << __FILE__ << ":" << __LINE__ << ": " << #x << " = " << ( x ) << endl

#include <complex>

typedef complex<double> Point;

const double EPS = 1e-9;

bool eq( double a, double b )
{
	return abs( a - b ) < EPS;
}

bool eq( Point a, Point b )
{
	return eq( a.real(), b.real() ) && eq( a.imag(), b.imag() );
}

double getRadius( double a, double b, double c )
{
	return ( a * b * c ) / sqrt( ( a + b + c ) * ( -a + b + c ) * ( a - b + c ) * ( a + b - c ) );
}

double fix( double a )
{
	return abs( a ) < EPS ? 0 : a;
}

void solve( Point a, Point b, Point c )
{
	double ab = abs( a - b );

	Point unit_ab = ( b - a ) / ab;
	Point mid_ab = Point( ( b - a ).real() / 2, ( b - a ).imag() / 2 );
	Point norm_ab1 = Point( -( unit_ab.imag() ), unit_ab.real() ), norm_ab2 = Point( unit_ab.imag(), -( unit_ab.real() ) );

	double r = getRadius( abs( a - b ), abs( b - c ), abs( c - a ) );
	double t = sqrt( pow( r, 2 ) - pow( ab / 2, 2 ) );

	Point o1 = mid_ab + norm_ab1 * t, o2 = mid_ab + norm_ab2 * t;

	if ( !eq( abs( o1 - c ), r ) )
	{
		swap( o1, o2 );
	}

	cout << fix( o1.real() ) << ' ' << fix(  o1.imag() ) << ' ' << fix( r ) << endl;

	return;
}

int main()
{
	cin.tie( 0 );
	ios::sync_with_stdio( false );

	cout.precision( 3 );
	cout << fixed;

	int n;
	cin >> n;

	REP( i, 0, n )
	{
		double a, b, c, d, e, f;
		cin >> a >> b >> c >> d >> e >> f;

		solve( Point( a, b ), Point( c, d ), Point( e, f ) );
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
#include <string>
#include <set>
#include <cmath>

// x^2+y^2+ax+by+c=0

int main(int,char**){
	std::cout<< std::setprecision(3) << std::setiosflags(std::ios::fixed);
	int n;
	std::cin >> n;
	while(n--){
		double x1,y1,x2,y2,x3,y3,a,b,c;
		std::cin >>  x1 >> y1 >> x2 >> y2 >> x3 >>y3;
		double vec[]={ -x1*x1-y1*y1, -x2*x2-y2*y2, -x3*x3-y3*y3};
		double det(x1*y1+x2*y3+x3*y1-x1*y3-x2*y1-x3*y2);
		double inv[3][3] = { 
			{ y2-y3, -(y1-y3), y1-y2 },
			{ -(x2-x3), x1-x3, -(x1-x2) },
			{ x2*y3-x3*y2, -(x1*y3-x3*y1), x1*y2-x2*y1}
		};
		a= (inv[0][0]*vec[0] +inv[0][1]*vec[1] +inv[0][2]*vec[2])/det;
		b= (inv[1][0]*vec[0] +inv[1][1]*vec[1] +inv[1][2]*vec[2])/det;
		c= (inv[2][0]*vec[0] +inv[2][1]*vec[1] +inv[2][2]*vec[2])/det;
		std::cout<< 0-a/2 << ' ' << 0-b/2<<' ' << std::sqrt((a*a+b*b)/4-c)<<std::endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cmath>

using namespace std;

int main()
{
	int n;
	double x1,y1,x2,y2,x3,y3,xp,yp,r;
	double a,b,c,d,e,f,L,M,N;
	
	cin>>n;
	for (int i=0; i<n; i++) {
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		
		a=x1-x2;
		b=y1-y2;
		c=-1*(x1*x1+y1*y1)+(x2*x2+y2*y2);
		d=x1-x3;
		e=y1-y3;
		f=-1*(x1*x1+y1*y1)+(x3*x3+y3*y3);
		
		if (a==0) {
			M=c/b;
			L=(f-e*M)/d;
		}
		else{
		M=(c*d-a*f)/(b*d-a*e);//
		L=(c-b*M)/a;
		}
		N=-L*x1-M*y1-x1*x1-y1*y1;
		
		xp=-1*(L/2);
		yp=-1*(M/2);
		r=sqrt(-1*N+(L*L)/4+(M*M)/4);
			
		printf("%.3f %.3f %.3f\n",xp,yp,r);
	}
	
	return 0;	
}