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 <iomanip>
#include <cmath>

// Vector 2D
template<class T>
struct Vec2D
{
    T x,y;
    explicit Vec2D(T x_=T(), T y_=T()):x(x_),y(y_){}
    Vec2D(const std::pair<T,T>& p):x(p.first),y(p.second){}
    Vec2D& operator+=(const Vec2D& v){ return *this = *this+v; }
    Vec2D& operator-=(const Vec2D& v){ return *this = *this-v; }
    friend Vec2D operator+(const Vec2D& l, const Vec2D& r){ return Vec2D(l.x+r.x,l.y+r.y); }
    friend Vec2D operator-(const Vec2D& l, const Vec2D& r){ return Vec2D(l.x-r.x,l.y-r.y); }
    friend T norm(const Vec2D& v){ return std::hypot(v.x,v.y); }
    friend T norm2(const Vec2D& v){ return v.x*v.x+v.y*v.y; }
    friend T dot(const Vec2D& l, const Vec2D& r){ return l.x*r.x+l.y*r.y; }
    friend T cross(const Vec2D& l, const Vec2D& r){ return l.x*r.y-l.y*r.x; }
    friend int dir(const Vec2D& l, const Vec2D& r)
    {
        // assert(norm2(l)!=0);
        const auto c = cross(l,r);
        if(c>0) return 1;
        if(c<0) return -1;
        if(dot(l,r)<0) return -2;
        if(norm2(l)<norm2(r)) return 2;
        return 0;
    }
};

// Triangle 2D
template<class T=double>
class Triangle2D
{
    Vec2D<T> a,b,c;
public:
    explicit Triangle2D(T ax=T(), T ay=T(), T bx=T(), T by=T(), T cx=T(), T cy=T()):a(ax,ay),b(bx,by),c(cx,cy){}
    Triangle2D(const Vec2D<T>& a_, const Vec2D<T>& b_, const Vec2D<T>& c_):a(a_),b(b_),c(c_){}
    T area() const
    {
        return std::abs(a.x*(b.y-c.y)+b.x*(c.y-a.y)+c.x*(a.y-b.y))/2;
    }
    T dist_a() const { return norm(b-c); }
    T dist_b() const { return norm(c-a); }
    T dist_c() const { return norm(a-b); }
    Vec2D<T> circumcenter() const
    {
        const Vec2D<T> vb(2*(b.x-a.x),2*(c.x-a.x)),
                       vc(2*(b.y-a.y),2*(c.y-a.y)),
                       vd(norm2(a)-norm2(b),norm2(a)-norm2(c));
        return Vec2D<T>(cross(vc,vd)/cross(vb,vc),
                        cross(vd,vb)/cross(vb,vc));
    }
    T R() const { return dist_a()*dist_b()*dist_c()/(4*area()); }
};

static void solve()
{
    typedef double F;
    F x1,y1,x2,y2,x3,y3; std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    const Triangle2D<F> tri(x1,y1,x2,y2,x3,y3);
    auto c = tri.circumcenter();
    std::cout << c.x << " " << c.y << " " << tri.R() << std::endl;
}

int main()
{
    std::cin.tie(0);
    std::ios_base::sync_with_stdio(false);
    std::cout << std::fixed << std::setprecision(3);
    int n; std::cin >> n;
    for(int i=0; i<n; ++i) solve();
}

C++: #include <iostream>
#include <cmath>
#include <utility>
using namespace std;


class vec{
public:
	double x;
	double y;
	double normEXP2() const{
		return x*x + y*y;
	}
	vec& operator+(const vec& v){
		x += v.x;
		y += v.y;
		return *this;
	}
	vec& operator-(const vec& v){
		x -= v.x;
		y -= v.y;
		return *this;
	}
	vec& operator*(double scala){
		x *= scala;
		y *= scala;
		return *this;
	}
	double operator*(const vec& v) const{
		return x*v.x + y*v.y;
	}
};


istream& operator>>(istream& i, vec& v){
	i >> v.x >> v.y;
	return i;
}


pair<double, double> cramer(const vec& a, const vec& b){
	pair<double, double> solve;
	solve.first = (2 * a.normEXP2()*b.normEXP2() - 2 * b.normEXP2()*(a*b)) / (4 * a.normEXP2()*b.normEXP2() - 4 * ((a*b)*(a*b)));
	solve.second = (2 * a.normEXP2()*b.normEXP2() - 2 * a.normEXP2()*(a*b)) / (4 * a.normEXP2()*b.normEXP2() - 4 * ((a*b)*(a*b)));

	return solve;
}


int main(){
	int n;
	vec v1, v2, v3;
	vec b, c;
	vec solveVec;
	pair<double, double> solve;
	double x, y;
	double r;

	cin >> n;
	for (int i = 0; i < n; i++){
		cin >> v1 >> v2 >> v3;
		b = v2 - v1;
		c = v3 - v1;
		
		solve = cramer(b, c);
		solveVec = (b*solve.first) + (c*solve.second);

		solveVec = v1 + solveVec;
		r = sqrt(solveVec.normEXP2());

		y = (double)((int)((solveVec.y * 1000) + 0.5)) / 1000;
		r = (double)((int)((r * 1000) + 0.5)) / 1000;
		x = (double)((int)((solveVec.x * 1000) + 0.5)) / 1000;

		printf("%.3lf %.3lf %.3lf\n", x, y, r);
	}

	return 0;
}