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 <cmath>
#include <iostream>
#include <iomanip>

struct TPoint
{
    double x;
    double y;
};
struct TTriangle
{
    TPoint p1;
    TPoint p2;
    TPoint p3;
};
void SolveEquation(double a, double b, double c, double d, double e, double f, double* x, double* y)
{
    *y = (f - d * c / a)/( e - d * b / a);
    *x = (c - b * *y) / a;
}
/*
A(a, b) B(c, d) C(e, f)
(OA)^2 = (OB)^2
(OA)^2 = (OC)^2
x2-2ax+a2 + y2-2by+b2 = x2-2cx+c2 + y2-2dy+d2
(2c-2a)x + (2d-2b)y = c^2 + d^2 - a^2 - b^2;

x2-2ax+a2 + y2-2by+b2 = x2-2ex+e2 + y2-2fy+f2
(2e-2a)x + (2f-2b)y = e^2 + f^2 - a^2 - d^2
*/
TPoint GetCentral(const TTriangle& Tri)
{
    using std::pow;
    double a, b, c, d, e, f;
    a = 2 * (Tri.p2.x - Tri.p1.x);
    b = 2 * (Tri.p2.y - Tri.p1.y);
    c = pow(Tri.p2.x,2) + pow(Tri.p2.y,2) - pow(Tri.p1.x,2) - pow(Tri.p1.y,2);
    d = 2 * (Tri.p3.x - Tri.p1.x);
    e = 2 * (Tri.p3.y - Tri.p1.y);
    f = pow(Tri.p3.x,2) + pow(Tri.p3.y,2) - pow(Tri.p1.x,2) - pow(Tri.p1.y,2);
    TPoint P;
    SolveEquation(a, b, c, d, e, f, &P.x, &P.y);
    return P;
}
double GetLength(const TPoint& A, const TPoint& B)
{
    // return std::sqrt(std::pow(A.x - B.x, 2) + std::pow(A.y - B.y, 2));
    return std::sqrt((A.x - B.x)*(A.x - B.x) + (A.y - B.y)*(A.y - B.y));
}
double GetRadius(double a, double b, double c)
{
    return (a*b*c)/std::sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
}
double GetRadius(const TTriangle& Tri)
{
    double a = GetLength(Tri.p1, Tri.p2);
    double b = GetLength(Tri.p2, Tri.p3);
    double c = GetLength(Tri.p3, Tri.p1);
    return GetRadius(a, b, c);
}
int main()
{
    int n;
    std::cin >> n;
    for(int i = 0; i < n; ++i){
        // double a, b, c;
        // std::cin >> a >> b >> c;
        // std::cout << GetRadius(a, b, c) << std::endl;
        
        TTriangle tri;
        std::cin >> tri.p1.x >> tri.p1.y >> tri.p2.x >> tri.p2.y >> tri.p3.x >> tri.p3.y;
        TPoint C = GetCentral(tri);
        double r = GetRadius(tri);
        std::cout << std::fixed << std::setprecision(3);
        std::cout << C.x << " " << C.y << " " << r << std::endl;
    }
    return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<list>
#include<map>
#include<vector>
#include<algorithm>
#include<string.h>
#include<iomanip>
#include<math.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << x << endl;
using namespace std;

int main(){
    double x1, y1, x2, y2, x3, y3;
    double a1, a2, b1, b2, c1, c2;
    double xans, yans, r;
    int n;

    cin >> n;
    rep(i,n){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        a1 = 2 * (x2 - x1);
        b1 = 2 * (y2 - y1);
        c1 = x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2;
        a2 = 2 * (x3 - x1);
        b2 = 2 * (y3 - y1);
        c2 = x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3;
        xans = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
        yans = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
        r = sqrt( (x1 - xans) * (x1 - xans) + (y1 - yans) * (y1 - yans) );
        cout << fixed;
        cout << setprecision(3) << xans + 0.0005 << " " << setprecision(3) << yans + 0.0005 << " " << setprecision(3) << r + 0.0005 << endl;
    }
}