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>

using namespace std;
typedef struct { double x; double y;} point;
 
point getCircumcenter(point a, point b, point c)
{
    double A1 = 2 * ( b.x - a.x );
    double B1 = 2 * ( b.y - a.y );
    double C1 = pow(a.x,2)-pow(b.x,2) + pow(a.y,2)-pow(b.y,2);
    double A2 = 2 * ( c.x - a.x );
    double B2 = 2 * ( c.y - a.y );
    double C2 = pow(a.x,2)-pow(c.x,2) + pow(a.y,2)-pow(c.y,2);
    double X = (B1 * C2 - B2 * C1) / (A1 * B2 - A2 * B1);
    double Y = (C1 * A2 - C2 * A1) / (A1 * B2 - A2 * B1);
	point P;
	P.x=X;
	P.y=Y;
    return P;
}

double getTwoPointDictance(point a, point b)
{
	return fabs(sqrt(pow((a.x-b.x),2)+pow((a.y-b.y),2)));
}
double AntiMinusZero(double n)
{
	if(n==0)
	{
		return 0;
	}
	return n;
}

int main()
{
    int n;
    cin>>n;
    point a,b,c;
    while(n--)
	{
        cin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
        point ans = getCircumcenter(a,b,c);
        printf("%.3lf %.3lf %.3lf", AntiMinusZero(ans.x), AntiMinusZero(ans.y), getTwoPointDictance( a, ans));
    }
}

C++: #include <iostream>
#include <complex>
#include <cassert>
#include <vector>
#include <iomanip>

using namespace std;

typedef complex<double> P;

#define loop(i,a,b) for(int i=(a); i<(int)(b); i++)
#define rep(i,b) loop(i,0,b)

const double eps = 1e-10;

double cross(const P& a, const P& b) {
    return imag(conj(a)*b);
}
double dot(const P& a, const P& b) {
    return real(conj(a)*b);
}

struct L : public vector<P> {
    L(const P &a, const P &b) {
        push_back(a); push_back(b);
    }
};

P crosspoint(const L &l, const L &m) {
    double A = cross(l[1] - l[0], m[1] - m[0]);
    double B = cross(l[1] - l[0], l[1] - m[0]);
    if (abs(A) < eps && abs(B) < eps) return m[0]; // same line
    if (abs(A) < eps) assert(false); // !!!PRECONDITION NOT SATISFIED!!!
    return m[0] + B / A * (m[1] - m[0]);
}

int main(){
    cout << setprecision(3) << fixed;
    P ps[3];
    int n; cin>>n;
    rep(tt,n){
        rep(i,3){
            double x,y; cin>>x>>y;
            ps[i] = P(x,y);
        }
        P mid1 = (ps[0]+ps[1])/2.0;
        P mid2 = (ps[0]+ps[2])/2.0;

        L l1(mid1, mid1+(ps[0]-ps[1])*P(0,1));
        L l2(mid2, mid2+(ps[0]-ps[2])*P(0,1));

        P cp = crosspoint(l1,l2);

        cout << cp.real() << " " << cp.imag() << " " << abs(cp-ps[0]) << endl;
    }
}

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;
}