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>
using namespace std;

int main(){
	int n;
	cin >> n;
	double x[n], y[n], r[n];
	for(int i = 0; i < n; i++){
		double x1, y1, x2, y2, x3, y3;
		double a, b, c, d, e, f;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a = 2*(x1-x2);
		b = 2*(y1-y2);
		c = 2*(x2-x3);
		d = 2*(y2-y3);
		e = x1*x1-x2*x2+y1*y1-y2*y2;
		f = x2*x2-x3*x3+y2*y2-y3*y3;
		x[i] = (d*e-b*f)/(a*d-c*d);
		y[i] = (a*f-c*e)/(a*d-b*c);
		r[i] = sqrt((x[i]-x1)*(x[i]-x1)+(y[i]-y1)*(y[i]-y1));
	}
	for(int i = 0; i < n; i++){
		cout <<  fixed << setprecision(3) << x[i] << " " << y[i] << " " << r[i] << endl;
	}
}

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

const double EPS = 1e-4;

double dist(double x1, double y1, double x2, double y2) {
  return sqrt(pow(x2-x1, 2) + pow(y2-y1, 2));
}

int main() {
  int n;
  double x[3], y[3], a, b, c, sa, ca;
  double px, py, r;
  cin >> n;
  for (int k=0;k<n;k++) {
    for (int i=0;i<3;i++) {
      cin >> x[i] >> y[i];
    }
    a = dist(x[1], y[1], x[2], y[2]);
    b = dist(x[0], y[0], x[1], y[1]);
    c = dist(x[0], y[0], x[2], y[2]);
    ca = (a*a - b*b - c*c) / (2*b*c);
    sa = sqrt(1 - ca*ca);
    r = a / sa / 2;

    for (px=-r;px<=r;px+=0.0001) {
      py = sqrt(r*r - px*px);
      if (abs(pow(px-x[0], 2) + pow(py-y[0], 2) - r*r) < EPS &&
          abs(pow(px-x[1], 2) + pow(py-y[1], 2) - r*r) < EPS &&
          abs(pow(px-x[2], 2) + pow(py-y[2], 2) - r*r) < EPS) {
        break;
      }
    }
    printf("%.3f %.3f %.3f\n", px, py, r);
  }
  return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdip>
#define S second
#define F first
using namespace std;
typedef pair<double,double> P;

double Length(P p,P q){
  return sqrt(pow(p.first - q.first,2)+pow(p.second-q.second,2));
}

int main(){
  P p[3];
  double a,b,c,A,B,C,R,x,y;
  int n;
  cin >> n;
  while(n-- > 0){ 
    cin >> p[0].first >> p[0].second >> p[1].first >>p[1].second >> p[2].first >> p[2].second;
    a = Length(p[0],p[1]);
    b = Length(p[1],p[2]);
    c = Length(p[2],p[0]);
    A = acos((b*b+c*c-a*a)/(2*b*c));
    B = acos((a*a+c*c-b*b)/(2*a*c));
    C = acos((a*a+b*b-c*c)/(2*a*b));
    R = 0.5 * (a/sin(A));
    //x = 0.5*((((pow(p[0].F,2)+pow(p[0].S,2))*(p[1].S-p[2].S)+((pow(p[1].F,2)+pow(p[1].F,2))*(p[2].S-p[0].S))+((pow(p[2].F,2)+pow(p[2].S,2))*(p[0].S-p[1].S))) )   / (p[0].F*(p[1].S-p[2].S)+p[1].F*(p[2].S-p[0].S)+p[2].F*(p[0].S-p[1].S)));
    x = 0.5*( (pow(p[0].F,2)+pow(p[0].S,2))*(p[1].S-p[2].S) + (pow(p[1].F,2)+pow(p[1].S,2))*(p[2].S-p[0].S) + (pow(p[2].F,2)+pow(p[2].S,2))*(p[0].S-p[1].S)                           )/(       
																							 p[0].F*(p[1].S-p[2].S) + p[1].F*(p[2].S-p[0].S) + p[2].F*(p[0].S-p[1].S)            );

    y = 0.5*((pow(p[0].F,2)+pow(p[0].S,2))*(p[1].F-p[2].F)+(pow(p[1].F,2)+pow(p[1].S,2))*(p[2].F-p[0].F)+(pow(p[2].F,2)+pow(p[2].S,2))*(p[0].F-p[1].F))/(p[0].S*(p[1].F-p[2].F)+p[1].S*(p[2].F-p[0].F)+p[2].S*(p[0].F-p[1].F));

    printf("%.3f %.3f %.3f\n",x,y,R);
  }
  return 0;
}