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;
    cin >> num;
    double x1, x2, x3, y1, y2, y3;
    double xp, yp, r;

    while(cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3)
    {
        xp = (1/2.0)*((x1*x1+y1*y1)*(y3-y2)+(x2*x2+y2*y2)*(y1-y3)+(x3*x3+y3*y3)*(y2-y1))/(x1*(y3-y1)+x2*(y1-y3)+x3*(y2-y1));
        yp = -(x3-x2)/(y3-y2)*xp+(1/2.0)*(x3*x3-x2*x2+y3*y3-y2*y2)/(y3-y2);
        r = sqrt((x1 - xp)*(x1 - xp) + (y1 - yp)*(y1 - yp));

        cout << fixed << setprecision(3) << xp << " " << yp << " " << r << endl;
    }
}

C++: #include<stdio.h>
#include<math.h>

int FLG = 0;

typedef struct{
	double x,y;
}POINT;
	
POINT point(POINT p1,POINT p2);
double incl(POINT p1,POINT p2,int);
double cept(double x,double y,double a);

int main(){
	POINT p1,p2,p3,lp1,lp2;
	int n;
	double la1,la2,lb1,lb2;
	double x,y,r,tmp;
	
	scanf("%d",&n);	
	
	while(n>0){
	scanf("%lf %lf %lf %lf %lf %lf",&p1.x,&p1.y,&p2.x,&p2.y,&p3.x,&p3.y);
	
	lp1 = point(p1,p2);
	la1 = incl(p1,p2,1);
	lb1 = cept(lp1.x,lp1.y,la1);
	
	lp2 = point(p1,p3);
	la2 = incl(p1,p3,2);
	lb2 = cept(lp2.x,lp2.y,la2);
	
	if(FLG == 0){
		x = -1*(lb1-lb2)/(la1-la2);
		y = la1*x+lb1;
		tmp = ((x - p1.x)*(x - p1.x))+((y - p1.y)*(y - p1.y));
		r = sqrt(tmp);
	}
	
	
	//la1が無限
	else if(FLG == 1){
		
		x = lp1.x;
		y = la2 * x + lb2;
		tmp = ((x - p1.x)*(x - p1.x))+((y - p1.y)*(y - p1.y));
		r = sqrt(tmp);		
		
	}
	
	else if(FLG == 2){
		
		x = lp2.x;
		y = la1 * x + lb1;
		tmp = ((x - p1.x)*(x - p1.x))+((y - p1.y)*(y - p1.y));
		r = sqrt(tmp);		
		
	}
	
	
	
	printf("%.3f %.3f %.3f\n",x,y,r);
	
	n--;
	
	}
	
	return 0;
}

//中点座標
POINT point(POINT p1,POINT p2){
	POINT P;
	
	P.x = (p1.x + p2.x)/2;
	P.y = (p1.y + p2.y)/2;
	
	return (P);
}

//傾き
double incl(POINT p1,POINT p2,int plus){
	double a;
	
	double tmp;
	
	tmp = p2.x - p1.x;
	
	if(tmp != 0) a = (p2.y - p1.y)/tmp;
	
	else if(p2.y - p1.y == 0){
		
		FLG += plus;
		return a;
	}	
	
	else return 0;
		
	if(a==0) FLG += plus;
	else{
	a = -1*(1/a);
	}
	
	return a;
}

//切片
double cept(double x,double y,double a){
	double b;
	
	b = y-(a*x);
	
	return (b);
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>
#include <memory.h>
#include <map>
#include <math.h>
#include <utility>


#pragma warning(disable:4996)
using namespace std;
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;
const double PI = 3.141592653589793238;
const Int INF = 999999999999;
typedef pair<int, int> P;


int main()
{
	int n;
	cin >> n;
	REP(i, n){
		double x1, x2, x3, y1, y2, y3;
		cin >> x1>> y1 >> x2 >> y2 >> x3 >> y3;

		double x21 = x2 - x1;
		double x31 = x3 - x1;
		double y21 = y2 - y1;
		double y31 = y3 - y1;
		
		double xc = 1.0*((y31*(x21*x21 + y21*y21)) - (y21*(x31*x31 + y31*y31))) /
			(2 * (x21*y31 - y21*x31));

		double yc = 1.0*((x21*(x31*x31 + y31*y31)) - (x31*(x21*x21 + y21*y21))) /
			(2 * (x21*y31 - y21*x31));	
		

		double d = (xc - x1)*(xc - x1) + (yc - y1)*(yc - y1);
		d = sqrt(d);

		printf("%.3lf %.3lf %.3lf\n", xc, yc, d);
	}
	return 0;
}