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 marume( double x ){
	long long t = x * 1000 + 0.5;
	return t/1000.0;
}

int main(){
	int n;
	double tx, ty, x1, y1, x2, y2;
	
	cin >> n;
	while(n--){
		cin >> tx >> ty >> x1 >> y1 >> x2 >> y2;
		x1 -= tx; y1 -= ty;
		x2 -= tx; y2 -= ty;
		
		double dist1 = x1 * x1 + y1 * y1;
		double dist2 = x2 * x2 + y2 * y2;
		x1 *= 2; y1 *= 2;
		y1 *= 2; y2 *= 2;
		
		double detA = x1 * y2 - x2 * y1;
		double detX = dist1 * y2 - y1 * dist2;
		double detY = x1 * dist2 - dist1 * x1;
		double x = detX / detA;
		double y = detY / detA;
		double r = hypot(x, y);
		
		x = marume(x);
		y = marume(y);
		r = marume(r);
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}

C++:  /*
Write a program which prints the central coordinate (px, py) and the radius r of a circumscribed circle of a triangle which is constructed by three points (x1, y1)(x2, y2)(x3, y3) on the plane surface.
 */

#include <iostream>
#include <iomanip>
#include <cmath>
#include <vector>

using namespace std;

 /*
http://zo3kirin3.net/?p=297
http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q104383420
http://www.nn.iij4u.or.jp/~hsat/misc/math/centre/circumcentre.html
 */

int main() {
	int n; cin >> n;
	vector<double> x(3), y(3), x2(3), y2(3);		// x,yªÀWÌXgAx2,y2ªÀWÌñæÌXg
	
	for (int p=0; p<n; p++) {
		cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
		for (int i=0; i<3; i++) {
			x2[i] = x[i]*x[i];
			y2[i] = y[i]*y[i];
		}
		
		// eÓÌ·³ðßéBO½ûÌèðp
		double a, b, c, a2, b2, c2;
		a2 = abs((x[2]-x[1])*(x[2]-x[1]) + (y[2]-y[1])*(y[2]-y[1]));
        b2 = abs((x[0]-x[2])*(x[0]-x[2]) + (y[0]-y[2])*(y[0]-y[2]));
        c2 = abs((x[1]-x[0])*(x[1]-x[0]) + (y[1]-y[0])*(y[1]-y[0]));
		a = sqrt(a2); b = sqrt(b2); c = sqrt(c2);
		
		// ÊÏBwÌö®ðpB
		double s, S2;
		s = (a+b+c) / 2;
		S2 = s+(s-a)+(s-b)+(s-c);
		
		// OSðßéBö®ÍWeby[WðQÆ©xNgâÁÄÈ¢©çà¾Å«È¢c
		double px, py;
		px = (a2*(b2+c2-a2)*x[0] + b2*(c2+a2-b2)*x[1] + c2*(a2+b2-c2)*x[2]) / (16 * S2);
        py = (a2*(b2+c2-a2)*y[0] + b2*(c2+a2-b2)*y[1] + c2*(a2+b2-c2)*y[2]) / (16 * S2);
        
        // oÍB®`µÄoÍ·éB¼aÍ¼üÌûö®ÅB
        cout << setprecision(3) << fixed << px << " " << py << " " << sqrt(abs((px-x[0])*(px-x[0])+(py-y[0])*(py-y[0]))) << endl;
	}
	
	return 0;
}