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 <vector>
#include <cmath>
using namespace std;
typedef vector <double> vd;
typedef vector <vd> vvd;

int equation(vvd &A){
    int n1=3;
	for (int i1=0; i1<n1; i1++){
		int j1=i1;
		if (A[j1][i1]==0)
			for (int j2=j1+1; j2<n1; j2++)
				if (A[j2][i1]!=0){
					for (int i3=0; i3<n1+1; i3++)
						swap(A[j2][i3], A[j1][i3]);
					break;
				}
		double tmp = A[j1][i1];
		for (int i2=0; i2<n1+1; i2++)
			A[j1][i2]/=tmp;

		for (int j2=0; j2<n1; j2++){
			if (j2==j1) continue;
			tmp = A[j2][i1];
			for (int i2=0; i2<n1+1; i2++)
				A[j2][i2] -= tmp * A[j1][i2];
		}
	}

    return 0;
}

int main(void){
    int n=0;
	double x=0, y=0, x0=0, y0=0, r=0;
	vvd A(3, vd(4));

	cin>>n;
	for (int i=0; i<n; i++){
		for (int j=0; j<3; j++){
			cin>>x>>y;
			A[j][0] = x;
			A[j][1] = y;
			A[j][2] = 1;
			A[j][3] = -(x*x + y*y);
		}
		equation(A);

		x0 = -A[0][3] / 2;
		y0 = -A[1][3] / 2;
		r = sqrt(pow(x-x0, 2) + pow(y-y0, 2));

		cout<<fixed<<setprecision(3);
		cout<<x0<<" "<<y0<<" "<<r<<endl;
	}
    return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
int main()
{
long n,i;
double x1,y1,x2,y2,x3,y3,Px,Py,r,a1,b1,c1,a2,b2,c2;
cin>>n;
for(i=0;i<n;i++){
cin>>x1>>y1>>x2>>y2>>x3>>y3;
a1=(x2-x1)*2;
b1=(y2-y1)*2;
c1=x1*x1-x2*x2+y1*y1-y2*y2;
a2=(x3-x1)*2;
b2=(y3-y1)*2;
c2=x1*x1-x3*x3+y1*y1-y3*y3;
Px=(b1*c2-b2*c1)/(a1*b2-a2*b1);
Py=(c1*a2-c2*a1)/(a1*b2-a2*b1);
r=sqrt((x1-x)*(x1-x)+(y1-y)*(y1-y));
printf("%.3f %.3f %.3f\n",Px,Py,r);
}
return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <cmath>
#include <complex>
#include <iomanip>
using namespace std;
#define rep(i,n) for (int i=0; i < (n); i++)

typedef double D;
typedef complex<D> P;
typedef pair<P, P> L;
typedef vector<P> VP;
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) {		// (2)
  return (conj(a)*b).Y;
}

P crosspointLL(P a1, P a2, P b1, P b2) { // (16)
  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() {
  P a1, a2, a3;
  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;
    a1 = P(x1, y1);
    a2 = P(x2, y2);
    a3 = P(x3, y3);
    P ans = circumcenter(a1, a2, a3);
    std::cout << fixed << setprecision(3) << ans.X << " " << ans.Y << " " << abs(ans-a1) << std::endl;
  }

}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
#define MAX_N 20
using namespace std;

void swap(double &x,double &y){
	double t;
	t=x;
	x=y;
	y=t;
}

double cut4(double x){
	double temp=x*pow(10,3);
	temp=double(int(temp+0.5));
	return temp*pow(10,-3);
}

int main(){
	int n;
	double x1,x2,x3,y1,y2,y3,px[MAX_N],py[MAX_N];
	double a,b,c,d,r[MAX_N];
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		if(x1==x2){
			swap(x2,x3);
			swap(y2,y3);
		}else if(x2==x3){
			swap(x1,x2);
			swap(y1,y2);
		}
		a=(y2-y1)/(x2-x1);
		c=(y3-y2)/(x3-x2);
		if(a&&c){
			a=-1/a;
			c=-1/c;
			b=(y1+y2)/2-a*(x1+x2)/2;
			d=(y2+y3)/2-c*(x2+x3)/2;
			px[i]=(b-d)/(c-a);
			py[i]=(b*c-a*d)/(c-a);
		}else if(!a){
			c=-1/c;
			d=(y2+y3)/2-c*(x2+x3)/2;
			px[i]=(x1+x2)/2;
			py[i]=c*(x1+x2)/2+d;
		}else if(!c){
			a=-1/a;
			b=(y1+y2)/2-a*(x1+x2)/2;
			px[i]=(x2+x3)/2;
			py[i]=a*(x2+x3)/2+b;
		}

		r[i]=sqrt((px[i]-x1)*(px[i]-x1)+(py[i]-y1)*(py[i]-y1));	
	}
	for(int i=0;i<n;i++) printf("%.3lf %.3lf %.3lf\n",cut4(px[i]),cut4(py[i]),cut4(r[i])); 
	return 0;
}