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 <vector>
#include <algorithm>
#include <map>
#include <cmath>
#include <complex>
using namespace std;

typedef double D;
typedef complex<D> P;
const D EPS = 1e-9;
#define X real()
#define Y imag()
#define LE(n,m) ((n) < (m) + EPS)
#define GE(n,m) ((n) + EPS > (m))
#define EQ(n,m) (abs((n)-(m)) < EPS)

D cross(P a, P b) {
  return (conj(a)*b).Y;
}

P crosspointLL(P a1, P a2, P b1, P b2) {
  D d1 = cross(b2-b1, b1-a1);
  D d2 = cross(b2-b1, a2-a1);
  if (EQ(d1, 0) && EQ(d2, 0)) return a1;
  return a1 + d1/d2*(a2-a1);
}

P circumcenter(P a, P b, P c) {
  a = (a-c)*0.5;
  b = (b-c)*0.5;
  return c + crosspointLL(a, a*P(1,1), b, b*P(1,1));
}

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

    P a, b, c;
    a = P(x1, y1);
    b = P(x2, y2);
    c = P(x3, y3);
    P ansxy = circumcenter(a, b, c);
    std::cout << ansxy.X << " " << ansxy.Y << " " << hypot(ansxy.X-x1, ansxy.Y-y1) << std::endl;
  }

}

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

double sisya(double x)
{
	int a = static_cast<int>(x * 10000);
	x = static_cast<double>(static_cast<int>(x * 1000)) / 1000;
	if(a < 0 && a%10 <= -5)
		return x-0.001;
	else if(a%10 >= 5)
		return x+0.001;
	return x;
}

double dif(double x,double y,double xx, double yy)
{
	return sqrt((x-xx)*(x-xx)+(y-yy)*(y-yy));
}

int main()
{
	int n;
	double x1,x2,x3,y1,y2,y3;
	cin >> n;
	for(int i=0;i<n;i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double px = sisya(max(x1,max(x2,x3)) - (max(x1,max(x2,x3)) - min(x1,min(x2,x3))) / 2 );
		double py = sisya(max(y1,max(y2,y3)) - (max(y1,max(y2,y3)) - min(y1,min(y2,y3))) / 2 );
		double r  = sisya(dif(x1,y1,px,py));
		cout << setprecision(3) << fixed << px << " " << setprecision(3) << fixed << py << " " << setprecision(3) << fixed << r <<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <math.h>
using namespace std;
int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		double x[3], y[3];		// three points
		for( int i = 0; i < 3; i++ ){
			cin >> x[i] >> y[i];
		}
		double a[2];		// the slope of a line
		double b[2];		// the intersept of a line
		double ax, ay;
		int s = 0;
		for( int i = 0; i < 2; i++ ){
			if( y[i] == y[i+1] ){
				s = i+1;
			}else{
				a[i] = -(x[i] - x[i+1]) / (y[i] - y[i+1]);
				b[i] = (y[i] + y[i+1]) / 2.0 - a[i] * (x[i] + x[i+1]) / 2.0;
			}
		}
		if( a[0] == 0 ){
			ay = b[0];
			ax = ( ay - b[1] ) / a[1];
			if( s == 2){
				ax = ( x[1] + x[2] ) / 2;
				ay = a[0] * ax + b[0];
			}else if( s == 1 ){
				ax = ( x[0] + x[1] ) / 2;
				ay = a[1] * ax + b[1];
			}else{
				ax = ( b[1] - b[0] ) / ( a[0] - a[1] );
				ay = a[0] * ax + b[0];
			}
		}else if( a[1] == 0 ){
			ay = b[1];
			ax = ( ay - b[0] ) / a[0];
			if( s == 2){
				ax = ( x[1] + x[2] ) / 2;
				ay = a[0] * ax + b[0];
			}else if( s == 1 ){
				ax = ( x[0] + x[1] ) / 2;
				ay = a[1] * ax + b[1];
			}else{
				ax = ( b[1] - b[0] ) / ( a[0] - a[1] );
				ay = a[0] * ax + b[0];
			}
		}else{
			if( s == 2){
				ax = ( x[1] + x[2] ) / 2;
				ay = a[0] * ax + b[0];
			}else if( s == 1 ){
				ax = ( x[0] + x[1] ) / 2;
				ay = a[1] * ax + b[1];
			}else{
				ax = ( b[1] - b[0] ) / ( a[0] - a[1] );
				ay = a[0] * ax + b[0];
			}
		}
		double r = sqrt( pow( x[0] - ax, 2 ) + pow( y[0] - ay, 2 ) );
		printf("%.3f %.3f %.3f\n", ax, ay, r );
	}
}

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

int main()
{
  double eps = 10e-7;
  double x1,y1,x2,y2,x3,y3;
  scanf("%lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);
  double a = pow(x2-x3,2.0)+pow(y2-y3,2.0);
  double b = pow(x3-x1,2.0)+pow(y3-y1,2.0);
  double c = pow(x1-x2,2.0)+pow(y1-y2,2.0);
  double a2 = sqrt(pow(x2-x3,2.0)+pow(y2-y3,2.0));
  double b2 = sqrt(pow(x3-x1,2.0)+pow(y3-y1,2.0));
  double c2 = sqrt(pow(x1-x2,2.0)+pow(y1-y2,2.0));
  double s = (a2+b2+c2)/2.0;
  double S = sqrt(s*(s-a2)*(s-b2)*(s-c2));
  double px = (a*(b+c-a) * x1 + b*(c+a-b) * x2 + c*(a+b-c) * x3) / (16.0*pow(S,2.0));
  double py = (a*(b+c-a) * y1 + b*(c+a-b) * y2 + c*(a+b-c) * y3) / (16.0*pow(S,2.0));
  double r = a2*b2*c2/(4.0*S);
  px += 0.0005 + eps;
  py += 0.0005 + eps;
  r += 0.0005 + eps;
  printf("%.3f %.3f %.3f\n",px,py,r);
}