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 <utility>
#include <iostream>
#include <cmath>
#include <cstdio>

// ax + by = e1
// cx + dy = e2
auto solve (double a , double b , double c , double d , double e1 , double e2) -> std::pair <double , double>
{
    auto delta = a * d - b * c;
    auto x = (d * e1 - b * e2) / delta;
    auto y = (a * e2 - c * e1) / delta;
    return std::make_pair (std::move (x) , std::move (y));
}

auto distance (const std::pair <double , double> & a , const std::pair <double , double> & b) -> double
{
    auto dx = a.first  - b.first;
    auto dy = a.second - b.second;
    return std::sqrt (dx * dx + dy * dy);
}

auto show (double v) -> std::string
{
    if (-0.0005 < v && v < 0.0005)
    {
        return "0.000";
    }
    char s [100];
    std::sprintf (s , "%.3f" , v);
    return s;
}

auto main () -> int
{
    int n;
    std::cin >> n;
    for (auto i = 0; i < n; ++ i)
    {
        double x1 , y1 , x2 , y2 , x3 , y3;
        std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        auto b1 = (y2 - y1) * (y1 + y2) / 2 + (x2 - x1) * (x1 + x2) / 2;
        auto b2 = (y3 - y1) * (y1 + y3) / 2 + (x3 - x1) * (x1 + x3) / 2;
        auto p = solve ((x2 - x1) , (y2 - y1) , (x3 - x1) , (y3 - y1) , b1 , b2);
        auto r = distance (p , std::make_pair (x1 , y1));
        std::cout << show (p.first) << ' ' << show (p.second) << ' ' << show (r) << std::endl;
    }
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main(){
int k;
double x[3], y[3], a, b, c, d, e, f, l , m, n;
cin >> k;
while(k--){
cin>> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
a = 2 * (x[1] - x[0]);
b = 2 * (y[1] - y[0]);
c = ( (x[1]*x[1]) + (y[1]*y[1]) ) -( (x[0]*x[0]) + (y[0]*y[0]) );
d = 2 * (x[2] - x[0]);
e = 2 * (y[2] - y[0]);
f = ( (x[2]*x[2]) + (y[2]*y[2]) ) -( (x[0]*x[0]) + (y[0]*y[0]) );
l = (c*e - f*b) / (a*e - d*b);
m = (c*d - f*a) / (b*d - e*a);
n = sqrt((l - x[0])*(l - x[0]) + (m - y[0])*(m - y[0]));
cout << fixed <<setprecision(3) << l << " " << m << " " << n << endl;
}
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#define double long double
using namespace std;

double marume( double x ){
	long long t = x * 1000 + ((x > 0) ? 1 : -1) * 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;
		x2 *= 2; y2 *= 2;
		
		double detA = x1 * y2 - y1 * x2;
		double detX = dist1 * y2 - y1 * dist1;
		double detY = x1 * dist2 - dist2 * x1;
		double x = detX / detA; //if(x < 0){ x -= 1e-5; } else { x += 1e-5; }
		double y = detY / detA; //if(y < 0){ y -= 1e-5; } else { y += 1e-5; }
		double r = hypot(x, y) + 1e-7;
		
		x += tx;
		y += ty;
		
		x = marume(x);
		y = marume(y);
		r = marume(r);
		printf("%.3Lf %.3Lf %.3Lf\n", x, y, r);
	}
	return 0;
}

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

int main(void){
	int n;
	double x1, y1, x2, y2, x3, y3;
	double r, xp, yp;

	scanf("%d", &n);
	for(int i=0; i<n; i++){
		scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);

		xp = (1/2.0) * ((x1*x1 + y1*y1)*(y3 - y2) + (x2*x2 + y2*y2)*(y1 - y3) + (x3*x3 + y3*y3)*(y2 - y1)) / (x1*(y3 - y1) + x2*(y1 - y3) + x3*(y2 - y1));
		yp = -(x3 - x2)/(y3 - y2) * xp + (1/2.0) * (x3*x3 - x2*x2 + y3*y3 - y2*y2) / (y3 - y2);
		r = sqrt((x1 - xp)*(x1 - xp) + (y1 - yp)*(y1 - yp));

		printf("%.3f %.3f %.3f\n", xp, yp, r);
	}

	return 0;
}