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 <cstdio>
#include <string>
#include <sstream>

using namespace std;

long double sisya(long double a) {
	a *= 1000.0;
	//cout << a << endl;
	a += 0.5;
	a = (int)a;
	a /= 1000.0;
	return a; 
}

long double ll(long double x1, long double y1, long double x2, long double y2) {
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		long double x[3], y[3];
		for (int j = 0; j < 3; j++) {
			cin >> x[j] >> y[j];
		}
		long double a1, a2, b1, b2, c1, c2;
		a1 = 2.0 * (x[1]-x[0]);
		a2 = 2.0 * (x[2]-x[0]);
		b1 = 2.0 * (y[1]-y[0]);
		b2 = 2.0 * (y[2]-y[0]);
		c1 = x[0]*x[0]-x[1]*x[1]+y[0]*y[0]-y[1]*y[1];
		c2 = x[0]*x[0]-x[2]*x[2]+y[0]*y[0]-y[2]*y[2];
		long double X, Y, R;
		X = (b1*c2-b2*c1)/(a1*b2-a2*b1);
		Y = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		R = ll(X, Y, x[0], y[0]);
		//X = sisya(X);
		//Y = sisya(Y);
		//R = sisya(R);
		printf("%.3f %.3f %.3f\n", X, Y, R);
	}
	return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>
int main()
{
  int n;
  std::cin >> n;
  while (n--) {
    double x1, y1, x2, y2, x3, y3, px, py, r;
    std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double c = 2.0 * ((x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1));
    px = ((y3 - y1) * (x2 * x2 - x1 * x1 + y2 * y2 - y1 * y1) +
          (y1 - y2) * (x3 * x3 - x1 * x1 + y3 * y3 - y1 * y1)) / c;
    py = ((x1 - x3) * (x2 * x2 - x1 * x1 + y2 * y2 - y1 * y1) +
          (x2 - x1) * (x3 * x3 - x1 * x1 + y3 * y3 - y1 * y1)) / c;
    r = sqrt(pow(x1 - px, 2) + pow(y1 - py, 2));
    std::cout << std::fixed << std::setprecision(3) << px << " " <<
      py << " " << r << std::endl;
  }
  return 0;
}