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>

double round3(double d)
{
    return floor(d * 1000.0 + 0.5) / 1000.0;
}

int main()
{
    int n;
    std::cin >> n;

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

        double p = ((x1*x1-x2*x2+y1*y1-y2*y2)*(y1-y3)-(x1*x1-x3*x3+y1*y1-y3*y3)*(y1-y2))/(2*(x1-x2)*(y1-y3)-2*(x1-x3)*(y1-y2));
        double q = ((x1*x1-x2*x2+y1*y1-y2*y2)*(x1-x3)-(x1*x1-x3*x3+y1*y1-y3*y3)*(x1-x2))/(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));
        double r = sqrt((x1 - p) * (x1 - p) + (y1 - q) * (y1 - q));

        std::cout << std::fixed << std::setprecision(3) << round3(p) << " " << round3(q) << " " << round3(r) << std::endl;
    }
}

C++: #include<cstdio>
#include<cmath>
 
//#include<iostream>
using namespace std;
 
int main(){
    int n;
    float x1, y1, x2, y2, x3, y3, xp,yp, xq, yq, s1, sr1, s2, sr2;
    float b1, b2, xc, yc, r;
    scanf("%d", &n);
    while(n--){
        scanf("%f%f%f%f%f%f",&x1, &y1, &x2, &y2, &x3, &y3);
 
        if(!(y2-y1)){ 
            r=x1; x1=x3; x3=r; r=y1; y1=y3; y3=r;
        }
        xp=(x2+x1)/2; yp=(y2+y1)/2;
        xq=(x3+x1)/2; yq=(y3+y1)/2;
        sr1=-(x2-x1)/(y2-y1); b1=yp-sr1*xp;
        sr2=-(x3-x1)/(y3-y1); b2=yq-sr2*xq;
 
        xc=-(b1-b2)/(sr1-sr2);
        yc=sr1*xc+b1;
        r=sqrt((xc-x1)*(xc-x1)+(yc-y1)*(yc-y1));
        printf("%.3f %.3f %.3f\n",xc, yc, r);
    }
    return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
using namespace std;
typedef vector <double> vd;
typedef vector < vd > vvd;

vvd inverse(vvd x){
	vvd tmp(2, vd(2));
	if (x.size()!=2) return tmp;
	double a, b, c, d, e;
	a = x[0][0];
	b = x[0][1];
    c = x[1][0];
	d = x[1][1];
    e = 1.0 / (a*d-b*c);
	tmp[0][0] = d * e;
	tmp[0][1] = -b * e;
	tmp[1][0] = -c * e;
	tmp[1][1] = a * e;
	return tmp;
}

vvd Mmult(vvd A, vvd B){
    int n1=A.size();
    int n2=A[0].size();
	int n3=B[0].size();
	vvd C(n1, vd(n3));
	for (int i=0; i<n1; i++)
		for (int j=0; j<n3; j++)
			C[i][j] = 0;

    for (int i=0; i<n1; i++)
		for (int j=0; j<n2; j++)
			for (int k=0; k<n3; k++)
				C[i][k] += A[i][j] * B[j][k];
    return C;
}

int main(void){
    int n;
	double x1, y1, x2, y2, x3, y3, dx1, dy1, dx2, dy2;
	vvd A(2, vd(2)), B(2, vd(1));
	cin>>n;
	for (int i=1; i<n+1; i++){
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		dx1 = x2 - x1;
		dy1 = y2 - y1;
		dx2 = x3 - x1;
		dy2 = y3 - y1;
		A[0][0] = -dx1;
		A[0][1] = dy1;
		A[1][0] = -dx2;
		A[1][1] = dy2;
		B[0][0] = -dx1*(x1+x2)/2 + dy1*(y1+y2)/2;
		B[1][0] = -dx2*(x1+x3)/2 + dy2*(y1+y3)/2;
		B = Mmult(inverse(A), B);

		double r = pow(pow(B[0][0]-x1,2)+pow(B[1][0]-y1,2), 0.5);
	    cout<<fixed<<setprecision(3);
		cout<<B[0][0]<<" "<<B[1][0]<<" "<<r<<endl;
	}
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int main() {
	int n;
	cin>>n;
	while(n--){
		double x1,x2,x3,y1,y2,y3;
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
	 	double a,b,c;
	 	a=pow((x2-x3),2)+pow((y2-y3),2);
	 	b=pow((x3-x1),2)+pow((y3-y1),2);
	 	c=pow((x1-x2),2)+pow((y1-y2),2);
	 	double ox,oy;
	 	ox=(a*(b+c-a)*x1+b*(c-b+a)*x2+c*(a+b-c)*x3)/(2*(a*b+b*c+c*a)-(a*a+b*b+c*c));
	 	oy=(a*(b+c-a)*y1+b*(c-b+a)*y2+c*(a+b-c)*y3)/(2*(a*b+b*c+c*a)-(a*a+b*b+c*c));
	 	double r;
	 	r=hypot((ox-x1),(oy-y1));
		printf("%.3lf %.3lf %.3lf\n",ox,oy,r);
	}
	return 0;
}

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

using namespace std;

double pow2(double x);

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

        double xx1 = pow2(x1);
        double yy1 = pow2(y1);
        double xx2 = pow2(x2);
        double yy2 = pow2(y2);
        double xx3 = pow2(x3);
        double yy3 = pow2(y3);

        double a1 = 2 * (x2 - x1);
        double b1 = 2 * (y2 - y1);
        double c1 = xx1 - xx2 + yy1 - yy2;
        double a2 = 2 * (x3 - x1);
        double b2 = 2 * (y3 - y1);
        double c2 = xx1 - xx3 + yy1 - yy3;

        double xp = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
        double yp = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
        double r = sqrt(pow2(xp - x1) + pow2(yp - y1));

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

    return 0;
}

double pow2(double x)
{
    return (x * x);
}