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>
 
int main()
{
    using namespace std;
    int n;
 
    cin >> n;
    for( int i=0; i<n; ++i ) {
        double x1, y1, x2, y2, x3, y3;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
 
        double const p
            = ( ( y1 - y3 ) * ( y1 * y1 - y2 * y2 + x1 * x1 - x2 * x2 ) - ( y1 - y2 ) * ( y1 * y1 - y3 * y3 + x1 * x1 - x3 * x3 ) )
            / ( 2 * ( y1 - y3 ) * ( x1 - x2 ) - 2 * ( y1 - y2 ) * ( x1 - x3 ) );
 
        double const q
            = ( ( x1 - x3 ) * ( x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2 ) - ( x1 - x2 ) * ( x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3 ) )
            / ( 2 * ( x1 - x3 ) * ( y1 - y2 ) - 2 * ( x1 - x2 ) * ( y1 - y3 ) );
 
        double const r1 = sqrt( pow( p - x1, 2 ) + pow( q - y1, 2 ) );
        double const r2 = sqrt( pow( p - x2, 2 ) + pow( q - y2, 2 ) );
        double const r3 = sqrt( pow( p - x3, 2 ) + pow( q - y3, 2 ) );
        double const r = ( r1 + r2 + r3 ) / 3.0;
 
        cout << showpoint << fixed << setprecision( 3 );
        cout << p << " " << q << " " << r << endl;
    }
}

C++: #include <bits/stdc++.h>
using namespace std;

double eps = 1e-10;

class Point {
public:
	double x, y;
	
	Point(double x = 0, double y = 0) : x(x), y(y) {}
	
	Point operator + (const Point& p) {
		return Point(x + p.x, y + p.y);
	}
	Point operator - (const Point& p) {
		return Point(x - p.x, y - p.y);
	}
	Point operator * (const double& a) {
		return Point(x * a, y * a);
	}
	Point operator / (const double& a) {
		return Point(x / a, y / a);
	}
	
	bool operator < (const Point& p) const {
		return x != p.x ? x < p.x : y < p.y;
	}
	bool operator == (const Point &p) const {
		return fabs(x - p.x) < eps && fabs(y - p.y) < eps;
	}
	
	double abs() {
		return sqrt(norm());
	}
	double norm() {
		return x * x + y * y;
	}
	
};

typedef Point Vector;

bool equals(const double& a, const double& b) {
	return fabs(a - b) < eps;
}

double dot(const Vector& a, const Vector& b){
	return a.x * b.x + a.y * b.y;	
}

double cross(const Vector& a, const Vector& b){
	return a.x * b.y - a.y * b.x;
}

class Triangle {
public:
	Point p1, p2, p3; // ??????
	Vector v1, v2, v3; // ??????????????????
	double e1, e2, e3; // ????????§??????
	double a1, a2, a3; // ?§????(????????¢???)
	
	Triangle(Point a, Point b, Point c) : p1(a), p2(b), p3(c) {
		v1 = p3 - p2; v2 = p3 - p1; v3 = p2 - p1;
		e1 = v1.abs(); e2 = v2.abs(); e3 = v3.abs();
		a1 = law_of_cosines(e1, e2, e3); a2 = law_of_cosines(e2, e1, e3);
		a3 = law_of_cosines(e3, e2, e1);
	}
	
	// ????????????(?????????:?§????)
	double law_of_cosines(const double& a, const double& b, const double& c) { 
		return acos((b * b + c * c - a * a) / (2.0 * b * c));
	}
	
	double calc_radius_circumcenter() {
		return e1 / (2.0 * sin(a1));
	}
	// ?????????????????????
	Vector calc_vec_circumcenter() {
		return (v1 * sin(2.0 * a1) + v2 * sin(2.0 * a2) + v3 * sin(2.0 * a3))
		       / (sin(2.0 * a1) + sin(2.0 * a2) + sin(2.0 * a3));
	}
	
	// ???????????§?¨?
	Point calc_p_circumcenter() {
		return Point((p1.x * sin(2.0 * a1) + p2.x * sin(2.0 * a2) + p3.x * sin(2.0 * a3))
		             / (sin(2.0 * a1) + sin(2.0 * a2) + sin(2.0 * a3)),
		             (p1.y * sin(2.0 * a1) + p2.y * sin(2.0 * a2) + p3.y * sin(2.0 * a3))
		             / (sin(2.0 * a1) + sin(2.0 * a2) + sin(2.0 * a3)));
	}
};

int main() {
	int n;
	cin >> n;
	double x1, x2, x3, y1, y2, y3;
	for(int i = 0; i < n; ++i){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		Triangle t = Triangle(Point(x1, y1), Point(x2, y2), Point(x3, y3));
		Point p = t.calc_p_circumcenter();
		printf("%.3f %.3f %.3f\n", p.x, p.y, t.calc_radius_circumcenter());
	}
	return 0;
}