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;

struct point{
    double x;
    double y;
};

int main(){
    int n;
    struct point points[3];

    cin>>n;
    for (int j = 0; j < n; ++j) {
        double s,a,b,c;
        for (int i = 0; i < 3; ++i) {
            cin >> points[i].x >> points[i].y;
        }
        a  = sqrt(pow(points[0].x-points[1].x,2)+pow(points[0].y-points[1].y,2));
        b  = sqrt(pow(points[1].x-points[2].x,2)+pow(points[1].y-points[2].y,2));
        c  = sqrt(pow(points[2].x-points[0].x,2)+pow(points[2].y-points[0].y,2));
        s  = (a+b+c)/2;
        printf("%.3f %.3f %.3f\n",
               ((points[0].y - points[2].y)*(pow(points[0].y,2) - pow(points[1].y,2) + pow(points[0].x,2) - pow(points[1].x,2)) - (points[0].y - points[1].y)*(pow(points[0].y,2) - pow(points[2].y,2) + pow(points[0].x,2) - pow(points[2].x,2))) / (2*(points[0].y - points[2].y)*(points[0].x - points[1].x) - 2*(points[0].y - points[1].y)*(points[0].x - points[2].x))+5e-5,
               ((points[0].x - points[2].x)*(pow(points[0].x,2) - pow(points[1].x,2) + pow(points[0].y,2) - pow(points[1].y,2)) - (points[0].x - points[1].x)*(pow(points[0].x,2) - pow(points[2].x,2) + pow(points[0].y,2) - pow(points[2].y,2))) / (2*(points[0].x - points[2].x)*(points[0].y - points[1].y) - 2*(points[0].x - points[1].x)*(points[0].y - points[2].y))+5e-5,
               a*b*c/(4*sqrt(s*(s-a)*(s-b)*(s-c)))+5e-5
        );
    }
    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>

struct Vector2D {
	double x, y;

	Vector2D() : x(0.0), y(0.0) {}
	Vector2D(double vx, double vy) : x(vx), y(vy) {}
	Vector2D& operator+=(const Vector2D& rhs)
	{
		x += rhs.x;
		y += rhs.y;
		return *this;
	}
	Vector2D& operator-=(const Vector2D& rhs)
	{
		x -= rhs.x;
		y -= rhs.y;
		return *this;
	}
	Vector2D& operator*=(double rhs)
	{
		x *= rhs;
		y *= rhs;
		return *this;
	}
};

Vector2D operator+(const Vector2D& lhs, const Vector2D& rhs)
{
	Vector2D result(lhs);
	return result += rhs;
}

Vector2D operator-(const Vector2D& lhs, const Vector2D& rhs)
{
	Vector2D result(lhs);
	return result -= rhs;
}

Vector2D operator*(double lhs, const Vector2D& rhs)
{
	Vector2D result(rhs);
	return result *= lhs;
}

void solve(Vector2D p1, Vector2D p2, Vector2D p3, Vector2D& center, double& radius)
{
	// p3-base p1, p3-base p2
	Vector2D r1(p1 - p3), r2(p2 - p3);

	double det = r2.x * r1.y - (-r1.x) * r2.y;
	double a = (r2.x * (r2.x - r1.x) + r2.y * (r1.y - r2.y)) * 0.5 / det;

	center = p3 + (0.5 * r1 + a * Vector2D(r1.y, -r1.x));

	Vector2D r = p1 - center;
	radius = std::sqrt(r.x * r.x + r.y * r.y);
}

int main()
{
	int count = 0;
	std::cin >> count;
	for (int i = 0; i < count; ++i) {
		Vector2D p1,p2,p3;
		std::cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y;
		if (!std::cin.good()) {
			break;
		}
		Vector2D center;
		double radius = 0.0;
		solve(p1, p2, p3, center, radius);
		std::printf("%.3f %.3f %.3f\n", center.x, center.y, radius);
	}

	return 0;
}

C++: #include <stdio.h>
#include <math.h>

using namespace std;


int main() {
	int n;
	double x1, x2, x3, y1, y2, y3, ox, oy, r;
	scanf("%d", &n);

	for (int i = 0; i < n; i++) {
		scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
		ox = ((y2 - y1) * ((y3 * y3 - y1 * y1) + (x3 * x3 - x1 * x1)) - (y3 - y1) * ((y2 * y2 - y1 * y1) + (x2 * x2 - x1 * x1))) /
			(2 * ((x3 - x1) * (y2 - y1) - (x2 - x1) * (y3 - y1)));
		if (y2 != y1) {
			oy = (2 * (x1 - x2) * ox + (y2 * y2 - y1 * y1) + (x2 * x2 - x1 * x1)) / (2 * (y2 - y1));
		}
		else {
			oy = (2 * (x1 - x3) * ox + (y3 * y3 - y1 * y1) + (x3 * x3 - x1 * x1)) / (2 * (y3 - y1));
		}

		r = sqrt((ox - x1) * (ox - x1) + (oy - y1) * (oy - y1));
		printf("%.3lf %.3lf %.3lf\n", ox, oy, r);
	}
	return 0;
}