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 <bits/stdc++.h>

using namespace std;

#define EPS (1e-5)
#define equals(a, b) (fabs(a - b) < EPS)

struct Point {
    double x, y;

    Point(){}
    Point(double x, double y) : x(x), y(y) {}

    Point operator + (const Point &p) const {
        return Point(x + p.x, y + p.y);
    }

    Point operator - (const Point &p) const {
        return Point(x - p.x, y - p.y);
    }
    
    Point operator * (const double &k) const {
        return Point(x * k, y * k);
    }
};

typedef Point Vector;

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

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

double norm(const Point &p)
{
    return dot(p, p);
} 

double abs(const Point &p)
{
    return sqrt(norm(p));
}

struct Line {
    Point s, t;
    Line(){}
    Line(Point s, Point t) : s(s), t(t) {}
};

Line getPerpendicularBisector(const Point &a, const Point &b)
{
    double cx = (a.x + b.x) / 2.0;
    double cy = (a.y + b.y) / 2.0;
    Point p = Point(cx + (a.y - b.y), cy + (b.x - a.x));
    return Line(Point(cx, cy),p);
}

Point crosspointLL(const Line &a, const Line &b)
{
    Vector va = a.t - a.s, vb = b.t - b.s;
    double d = cross(vb, va);
    if (abs(d) < EPS) return b.s;
    return a.s+va*cross(vb, b.t - a.s) * (1.0 / d);
}

int main()
{
    Point p[3];
    int N;
    cin >> N;
    while (N--) {
        for (int i = 0; i < 3; i++) {
            cin >> p[i].x >> p[i].y;
        }
        Line l[2];
        for (int i = 0; i < 2; i++) {
            l[i] = getPerpendicularBisector(p[i], p[i+1]);
        }
        Point c = crosspointLL(l[0], l[1]);
        double r = abs(c - p[0]);
        printf("%.3f %.3f %.3f\n", c.x, c.y, r);
    }
    return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>

#define int64 long long int

using namespace std;

int main()
{
	int n;
	double x1,y1,x2,y2,x3,y3,x,y,r;
	double det,a1,b1,a2,b2,c1,c2;
	cin >> n;
	for(int i=0; i<n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a1 = 2 * (x1 - x2), b1 = 2 * (y1 - y2);
		a2 = 2 * (x2 - x3), b2 = 2 * (y2 - y3);
		det = a1 * b2 - a2 * b1;
		c1 = (x1*x1 - x2*x2) + (y1*y1 - y2*y2);
		c2 = (x2*x2 - x3*x3) + (y2*y2 - y3*y3);
		x = (a1 * c1 + b1 * c2) / det;
		y = (a2 * c1 + b2 * c2) / det;
		r = sqrt((x - x1)*(x - x1) + (y - y1)*(y - y1));
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}

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

double a, b, c, d, e, f;
double x, y;

double RoundNumber(double f) {
  int tf = (int)(10000 * f);
  if (tf % 10 >= 5) {
    tf += 10;
  }
  tf /= 10;
  return (double)tf / 1000.0f;
}

void solve(){
  /*
  double tmp1 = f-(c*d/a);
  double tmp2 = e-(b*d/a);
  y = tmp1/tmp2;
  x = (c - b * y) / a;
  */
  x = (c*e-b*f) / (a*e-b*d);
  y = (c*d-a*f) / (b*d-a*e);
  printf("%.3f %.3f ", RoundNumber(x), RoundNumber(y));        
}

int main(int argc, char const* argv[]) {
  int n;
  cin>>n;
  for (int i = 0; i < n; i += 1) {
    double x1, y1, x2, y2, x3, y3;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    a = -2*x1 + 2*x2;
    b = -2*y1 + 2*y2;
    c = x2*x2 + y2*y2 - x1*x1 - y1*y1;
    d = -2*x1 + 2*x3;
    e = -2*y1 + 2*y3;
    f = x3*x3 + y3*y3 - x1*x1 - y1*y1;

    solve();
    
    double r = (x - x1)*(x - x1) + (y - y1)*(y - y1);
    r = sqrt(r);
    printf("%.3f\n", RoundNumber(r));
  }  
  return 0;
}