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 <stdio.h>
#include <string>
#include <math.h>
#include <algorithm>
using namespace std;
int main(){
  double x1, y1, x2, y2, x3, y3;
  double px, py;
  double r;
  double M1, mx1, my1;
  double M2, mx2, my2;
  int n;
  cin >> n;

  while(1){
    if(n == 0) break;

    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

    mx1 = (x1 + x2) / 2;
    my1 = (y1 + y2) / 2;
    if(y1 - y2 == 0) M1 = 0;
    else  M1 = (x2 - x1) / (y1 - y2);
   

    mx2 = (x2 + x3) / 2;
    my2 = (y2 + y3) / 2;
    if(y2 - y3 == 0) M2 = 0;
    else  M2 = (x3 - x2) / (y2 -y3);
    

    if(M1 == M2){
      px = mx1;
      py = my2;
    }
    else if(M1 != M2){
      px = (M1*mx1 - my1 - M2*mx2 + my2) / (M1 - M2);
      py = M1*(px - mx1) + my1;
    }

    r = sqrt((x1 - px)*(x1 - px) + (y1 - py)*(y1 - py));

    printf("%.3f %.3f %.3f\n",px,py,r);
    n--;
  }
 return 0; 
}

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

// ax + by = e1
// cx + dy = e2
auto solve (double a , double b , double c , double d , double e1 , double e2) -> std::pair <double , double>
{
    auto delta = a * d - b * c;
    auto x = (d * e1 - b * e2) / delta;
    auto y = (a * e2 - c * e1) / delta;
    return std::make_pair (std::move (x) , std::move (y));
}

auto distance (const std::pair <double , double> & a , const std::pair <double , double> & b) -> double
{
    auto dx = a.first  - b.first;
    auto dy = a.second - b.second;
    return std::sqrt (dx * dx + dy * dy);
}

auto show (double v) -> std::string
{
    if (-0.5 < v && v < 0)
    {
        return "0.000";
    }
    char s [100];
    std::sprintf (s , "%.3f" , v);
    return s;
}

auto main () -> int
{
    int n;
    std::cin >> n;
    for (auto i = 0; i < n; ++ i)
    {
        double x1 , y1 , x2 , y2 , x3 , y3;
        std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        auto b1 = (y2 - y1) * (y1 + y2) / 2 + (x2 - x1) * (x1 + x2) / 2;
        auto b2 = (y3 - y1) * (y1 + y3) / 2 + (x3 - x1) * (x1 + x3) / 2;
        auto p = solve ((x2 - x1) , (y2 - y1) , (x3 - x1) , (y3 - y1) , b1 , b2);
        auto r = distance (p , std::make_pair (x1 , y1));
        std::cout << show (p.first) << ' ' << show (p.second) << ' ' << show (r) << std::endl;
    }
}

C++: #include <bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef double D;
typedef complex<D> P;
typedef vector<P> VP;
P minEnclosingCircle(const VP& ps) {
  P c;
  double move = 0.5;
  r(i,41) {  // 2^(-39-1) \approx 0.9e-12
    r(t,200) {
      D max = 0;
      int k = 0;
      r (j, ps.size()) if (max < norm(ps[j]-c)) {
        max = norm(ps[j]-c);
        k = j;
      }
      c += (ps[k]-c) * move;
    }
    move /= 2;
  }
  return c;
}
int main(){
    int n;
    cin>>n;
    r(i,n){
        VP p;
        D a,b;
        r(j,3){
            cin>>a>>b;
            p.push_back(P(a,b));
        }
        P t=minEnclosingCircle(p);
        D dis=abs(p[0]-t);
        printf("%.3f %.3f %.3f\n",t.real(),t.imag(),dis);
    }
}