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 <cstdio>
#include <cmath>
using namespace std;

double a, b, c, d, e, f;
double x, y;
double rx, ry;


double RoundNumber(double f) {
  int tf = (int)(10000 * f);
  if (tf % 10 >= 5) {
    tf += 10;
  }
  tf /= 10;
  return (double)tf / 1000.0f;
}

void solve(){
  double tmp1 = f-(c*d/a);
  double tmp2 = e-(b*d/a);
  y = tmp1/tmp2;
  x = (c - b * y) / a;  
  printf("%.3f %.3f ", RoundNumber(x), RoundNumber(y));        
}

int main(int argc, char const* argv[]) {
  int n;
  cin>>n;
  for (int i = 0; i < n; i += 1) {
    double x1, y1, x2, y2, x3, y3;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    a = -2*x1 + 2*x2;
    b = -2*y1 + 2*y2;
    c = x2*x2 + y2*y2 - x1*x1 - y1*y1;
    d = -2*x1 + 2*x3;
    e = -2*y1 + 2*y3;
    f = x3*x3 + y3*y3 - x1*x1 - y1*y1;

    solve();
    
    double r = (x - x1)*(x - x1) + (y - y1)*(y - y1);
    r = sqrt(r);
    printf("%.3f\n", RoundNumber(r));
  }  
  return 0;
}

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

double EPS = 1e-10;
const double PI = acos(-1);

using namespace std;

double add(double a, double b){
  if(abs(a+b) < EPS * (abs(a)+abs(b)))return 0;
  return a+b;
}

struct point{
  double x, y;
  point(){}
  point(double x,double y) : x(x) , y(y){}

  point operator + (point p){
    return point(add(x,p.x), add(y,p.y));
  }

  point operator - (point p){
    return point(add(x,-p.x), add(y,-p.y));
  }

  point operator * (double d){
    return point(x*d,y*d);
  }

  point operator / (double d){
    return point(x/d,y/d);
  }
  
  bool operator == ( const point &p ) const {
    return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS;
  }
};

struct circle{
  point p;
  double r;
};

circle circumscribed_circle(point p, point q, point r){
  double a[2],b[2],c[2];
  circle res;
    
    a[0]=2*(q.x-p.x);
    b[0]=2*(q.y-p.y);
    c[0]=p.x*p.x-q.x*q.x+p.y*p.y-q.y*q.y;
    
    a[1]=2*(r.x-p.x);
    b[1]=2*(r.y-p.y);
    c[1]=p.x*p.x-r.x*r.x+p.y*p.y-r.y*r.y;
    
    res.p.x=(b[0]*c[1]-b[1]*c[0])/(a[0]*b[1]-a[1]*b[0]);
    res.p.y=(c[0]*a[1]-c[1]*a[0])/(a[0]*b[1]-a[1]*b[0]);
 
    res.r=sqrt(pow(fabs(p.x-res.p.x),2)+pow(fabs(p.y-res.p.y),2));
    
    return res;
}

int main(void) {
  
  int n;
  point in[3];
  circle ans;
 
  cin >> n;
  
  while(n--){
    for(int i=0;i<3;i++)
      cin >> in[i].x >> in[i].y;

    ans=circumscribed_circle(in[0], in[1], in[2]);

    printf("%.3f %.3f %.3f\n",ans.p.x,ans.p.y,ans.r);
  }
  
  return 0;
}

C++: // includes
#include <cstdio>
#include <cstdint>
#include <iostream>
#include <iomanip>
#include <string>
#include <queue>
#include <stack>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <utility>
#include <functional>
#include <cmath>
#include <climits>
#include <bitset>

// macros
#define ll long long int
#define pb push_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a); i<(b);++i)
#define rep(i, n) FOR(i, 0, n)

using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;

// solve

int main(int argc, char const* argv[])
{
  int n;
  cin >> n;
  rep(i, n){
    double x[3], y[3];
    rep(j, 3){
      cin >> x[j] >> y[j];
    }
    int tmp = 0;
    while(x[tmp] == x[(tmp + 1) % 3] || x[tmp] == x[(tmp + 2) % 3]){
      swap(x[0], x[tmp]);
      swap(y[0], y[tmp]);
      tmp++;
    }
    double yc = (((x[0] - x[2]) * (x[0] * x[0] + y[0] * y[0] - x[1] * x[1] - y[1] * y[1])) \
                - ((x[0] - x[1]) * (x[0] * x[0] + y[0] * y[0] - x[2] * x[2] - y[2] * y[2]))) \
                / ((x[0] - x[2]) * (y[0] - y[1]) - (x[0] - x[1]) * (y[0] - y[2])) / 2.;
    double xc = (((y[0] - y[2]) * (x[0] * x[0] + y[0] * y[0] - x[1] * x[1] - y[1] * y[1])) \
                - ((y[0] - y[1]) * (x[0] * x[0] + y[0] * y[0] - x[2] * x[2] - y[2] * y[2]))) \
                / ((y[0] - y[2]) * (x[0] - x[1]) - (y[0] - y[1]) * (x[0] - x[2])) / 2.;
    cout << setprecision(3) << fixed;
    cout << xc << " " << yc << " ";
    cout << sqrt((x[0] - xc) * (x[0] - xc) + (y[0] - yc) * (y[0] - yc)) << endl;
  }
	return 0;
}