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 <cmath>
#include <iomanip>

#define ROUND_RANK 4
#define EPS 1e-8

using namespace std;

struct vec2
{
	double x;
	double y;
};

//行列式の算出
double Calc_det(const double& p11, const double& p12, const double& p21, const double& p22) {
	return (p11*p22 - p12*p21);
}
//xの解の算出
double Calc_x(const double& det, const double& q1, const double& p22, const double& p12, const double& q2) {
	return ((q1*p22 - p12*q2) / det);
}
//yの解の算出
double Calc_y(const double& det, const double& p11, const double& q2, const double& q1, const double& p21) {
	return ((p11*q2 - q1*p21) / det);
}
//rの解の算出
double Calc_r(const double& x0, const double& y0, const double& x1, const double& y1) {
	double r;
	r = sqrt(pow(x1 - x0, 2) + pow(y1 - y0, 2));
	return r;
}
//少数点以下第rank位で四捨五入
double Round(const double& value, const int& rank) {
	int root = pow(10, rank);
	return (std::round(value*root) / root);
}

int main() {
	int n;
	vec2 ver[20][3];

	cin >> n;
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < 3; ++j) {
			cin >> ver[i][j].x;
			cin >> ver[i][j].y;
		}
	}
	double p11, p12, p21, p22, q1, q2;
	double  det;//行列式
	double	x, y, r;//解
	for (int i = 0; i < n; ++i) {
		p11 = -2*(ver[i][0].x - ver[i][1].x);
		p12 = -2*(ver[i][0].y - ver[i][1].y);
		p21 = -2*(ver[i][1].x - ver[i][2].x);
		p22 = -2*(ver[i][1].y - ver[i][2].y);
		q1 = (ver[i][1].x + ver[i][0].x)*(ver[i][1].x - ver[i][0].x)
		   + (ver[i][1].y + ver[i][0].y)*(ver[i][1].y - ver[i][0].y);
		q2 = (ver[i][2].x + ver[i][1].x)*(ver[i][2].x - ver[i][1].x)
		   + (ver[i][2].y + ver[i][1].y)*(ver[i][2].y - ver[i][1].y);
		det = Calc_det(p11, p12, p21, p22);
		det = fabs(det);
		x = Calc_x(det, q1, p22, p12, q2);
		y = Calc_y(det, p11, q2, q1, p21);
		r = Calc_r(x, y, ver[i][0].x, ver[i][0].y);
		x = Round(x, ROUND_RANK);
		y = Round(y, ROUND_RANK);
		r = Round(r, ROUND_RANK);
		std::cout << std::fixed;
		std::cout << std::setprecision(3) << x + EPS << " " << y + EPS << " " << r + EPS <<endl;
	}
}

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

using namespace std;

int main(void){
	
	int n;
	double x[3]={0},y[3]={0},r,a,b,c,d[2]={0},e[2]={0},f[2]={0},l,m;

	cin>>n;

	for(int i=0;i<n;i++){
		
		a=b=c=r=l=m=0;
		for(int i=0;i<2;i++){
			d[i]=e[i]=f[i]=0;
		}
		for(int i=0;i<3;i++){
			x[i]=y[i]=0;
		}

		cin>>x[0]>>y[0]>>x[1]>>y[1]>>x[2]>>y[2];

		a=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));	//ツ各ツ陛督づ個陳キツつウ
		b=sqrt((x[2]-x[1])*(x[2]-x[1])+(y[2]-y[1])*(y[2]-y[1]));
		c=sqrt((x[0]-x[2])*(x[0]-x[2])+(y[0]-y[2])*(y[0]-y[2]));	
	
		r=a*b*c/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));			//ツ外ツ静堋円ツづ個板シツ径
		
		d[0]=2*(x[1]-x[0]);
		e[0]=2*(y[1]-y[0]);
		f[0]=x[0]*x[0]-x[1]*x[1]+y[0]*y[0]-y[1]*y[1];
		d[1]=2*(x[2]-x[0]);
		e[1]=2*(y[2]-y[0]);
		f[1]=x[0]*x[0]-x[2]*x[2]+y[0]*y[0]-y[2]*y[2];

		l=(e[0]*f[1]-e[1]*f[0])/(d[0]*e[1]-d[1]*e[0]);
		m=(f[0]*d[1]-f[1]*d[0])/(d[0]*e[1]-d[1]*e[0]);

		l=l*1000;
		l=l+0.5;
		l=int(l);
		l=l/1000;

		m=m*1000;
		m=m+0.5;
		m=int(m);
		m=m/1000;

		r=r*1000;
		r=r+0.5;
		r=(int)(r);
		r=r/1000;
		
		printf("%.03lf %.03lf %.03lf\n",l,m,r);
	}
	return 0;
}

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

using namespace std;

int main()
{
    int n;
    double x1, y1, x2, y2, x3, y3;
    double a1, a2, b1, b2, c1, c2;
    double x, y, r;

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

        a1 = 2 * (x2 - x1);
        b1 = 2 * (y2 - y1);
        c1 = sqrt(x1) - sqrt(x2) + sqrt(y1)  - sqrt(y2);
        a2 = 2 * (x3 - x1);
        b2 = 2 * (y3 - y1);
        c2 = sqrt(x1) - sqrt(x3) + sqrt(y1)  - sqrt(y3);

        x = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
        y = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
        r = pow(sqrt(abs(x - x1)) + sqrt(abs(y - y2)), 2);

        cout << setprecision(3) << fixed;
        cout << x << " " << y << " " << r << " " << endl;
    }

    return 0;
}