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>
using namespace std;
int main(){
	int k;
	double x[3], y[3], a, b, c, d, e, f, l , m, n;
	cin >> k;
	while(k--){
		cin>> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
		a = 2 * (x[1] - x[0]);
		b = 2 * (y[1] - y[0]);
		c = ( (x[1]*x[1]) + (y[1]*y[1]) ) -( (x[0]*x[0]) + (y[0]*y[0]) );
		d = 2 * (x[2] - x[0]);
		e = 2 * (y[2] - y[0]);
		f = ( (x[2]*x[2]) + (y[2]*y[2]) ) -( (x[0]*x[0]) + (y[0]*y[0]) );
		l = (c*e - f*b) / (a*e - d*b);
		m = (c*d - f*a) / (b*d - e*a);
		n = sqrt((l - x[0])*(l - x[0]) + (m - y[0])*(m - y[0]));
		cout << fixed << setprecision(3) << l << " " << m << " " << n << endl;
	}
}

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

void gradcut(double *a,double *b,double x1,double y1,double x2,double y2);
double fixzero(double z){
    if (z==-0) return 0;
    else return z;
}
int main() {
    double x[3],y[3],a[2],b[2],rx,ry,r;
    int n;
    std::cin >> n;
    for(int j=0;j<n;j++){
        int am=0,add=1;;
        for(int i=0;i<3;i++) std::cin >> x[i] >> y[i];
        for(int i=0;i<3;i++){
            if(i==2) add=-2;
            if(x[i]!=x[i+add]&&y[i]!=y[i+add]){
                gradcut(&a[am],&b[am],x[i],x[i+add],y[i],y[i+add]);
                am++;
                if(am>2) break;
            }else if(x[i]==x[i+add]){
                a[am]=0;
                b[am]=(y[i]+y[i+add])/2;
                am++;
                if(am>2) break;
            }
        }


        rx = (b[1]-b[0])/(a[0]-a[1]);
        ry = a[0]*rx+b[0];
        r=sqrt((x[0]-rx)*(x[0]-rx)+(y[0]-ry)*(y[0]-ry));
        printf("%.3f %.3f %.3f\n",fixzero(rx),fixzero(ry),fixzero(r));
    }
    return 0;
}

void gradcut(double *a,double *b,double x1,double x2,double y1,double y2){
    *a=-(x2-x1)/(y2-y1);
    *b=(y1+y2)/2-(*a)*(x1+x2)/2;
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>
struct {double x, y;} p[3];
using namespace std;
int main()
{
	int t, i, j;
	double r, a, b, c, d, e, f, l, m, n;
	cin >> t;
	for (i = 0; i < t; i++)
	{
		for (j = 0; j < 3; j++)
			cin >> p[j].x >> p[j].y;
		a = p[0].x-p[1].x;
		b = p[0].y-p[1].y;
		c = -(p[0].x*p[0].x)-(p[0].y*p[0].y)+(p[1].x*p[1].x)+(p[1].y*p[1].y);
		d = p[1].x-p[2].x;
		e = p[1].y-p[2].y;
		f = -(p[1].x*p[1].x)-(p[1].y*p[1].y)+(p[2].x*p[2].x)+(p[2].y*p[2].y);
		m = (c*d-a*f)/(b*d-a*e);
		l = (c*e-b*f)/(e*a-b*d);
		n = -((p[0].x*p[0].x)+(p[0].y*p[0].y)+p[0].x*l+p[0].y*m);
		cout << setprecision(3) << setiosflags(ios::fixed);
		cout << -l/2 << " " << -m/2 << " " << sqrt(l*l+m*m-4*n)/2 << endl;
	}
}

C++: #include <algorithm>
#include <functional>
#include <cmath>
#include <cstdio>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <bitset>
#include <climits>
#include <complex>
 
#define all(c) (c).begin(), (c).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define pb(e) push_back(e)
#define mp(a, b) make_pair(a, b)
#define fr first
#define sc second
 
const int INF=100000000;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
using namespace std;
typedef complex<double> P;
int n;
 
double cross(P p1,P p2) {
    return (real(p1)*imag(p2) - imag(p1)*real(p2));
}
double calc_S(P p1,P p2,P p3) {
    double ret = 0.0;
    p1 -= p3;
    p2 -= p3;
 
    ret = cross(p1,p2)/2.0;
     
    return abs(ret);
}
 
int main() {
    cin>>n;
    while(n-->0) {
        double x1_,y1_;
        double x2_,y2_;
        double x3_,y3_;
        cin>>x1_>>y1_>>x2_>>y2_>>x3_>>y3_;
        P p1(x1_,y1_);
        P p2(x2_,y2_);
        P p3(x3_,y3_);
        P A = p1-p2;
        P B = p2-p3;
        P C = p1-p3;
        double S = calc_S(p1,p2,p3);
        double R = abs(A)*abs(B)*abs(C)/S;
        R /= 4.0;
         
        P A1 = 2.0*(p2-p1);
        P B1 = 2.0*(p3-p1);
        P C1 = P(norm(p1)-norm(p2),norm(p1)-norm(p3));
        double X = cross(P(imag(A1),imag(B1)),C1)/cross(A1,B1);
        double Y = cross(C1,P(real(A1),real(B1)))/cross(A1,B1);
        printf("%.3lf %.3lf %.3lf\n",X,Y,R);
 
    }
    return 0;
}