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>
#include<cstdio>
using namespace std;


int main(void){

    int n;
  while(cin >> n){
    
    for(int i = 0;i < n;i++){

      double x1,y1,x2,y2,x3,y3;
      cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

      double a = sqrt(pow(x2-x1,2) + pow(y2-y1,2));
      double b = sqrt(pow(x3-x2,2) + pow(y3-y2,2));
      double c = sqrt(pow(x3-x1,2) + pow(y3-y1,2));

      double r = a*b*c / sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));

      double A = pow(x2-x1,2) + pow(y2-y1,2);
      double B = pow(x3-x2,2) + pow(y3-y2,2);
      double C = pow(x3-x1,2) + pow(y3-y1,2);
      double s = (A*(B+C-A) + B*(C+A-B) + C*(A+B-C));


      double x = 2*A*(B+C-A) / s;
      double y = 2*B*(C+A-B) / s;
      if(x == 0){
        x = 0;
      }
      if(y == 0){
        y=0;
      }

      printf("%.3f %.3f %.3f\n",x,y,r);

    }
  }




  return 0;
}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
#include <cassert>
using namespace std;
typedef pair<double, double> PDD;
PDD calc(double mat[2][2], double vec[2]){
  double a = mat[0][0], b = mat[0][1], d = mat[1][0], e = mat[1][1];
  double c = vec[0], f = vec[1];
  double det = a * e - d * b;
  return make_pair((e * c - b * f) / det, (-c * d + a * f) / det);
}
int main(){
  int Tc;
  cin >> Tc;
  for(int tc = 0; tc < Tc; ++tc){
    double x1, y1, x2, y2, x3, y3;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    if(y1 - y3 == 0){
      swap(y2, y3);
      swap(x2, x3);
    }
    else if(y2 - y3 == 0){
      swap(y1, y3);
      swap(x1, x3);
    }
    double A, B, C, D, E, F;
    A = (y1 + y3) / 2.;
    B = (x1 - x3) / (y1 - y3);    
    C = (x1 + x3) / 2.;
    D = (y2 + y3) / 2.;
    E = (x2 - x3) / (y2 - y3);
    F = (x2 + x3) / 2.;
    double mat[2][2];
    mat[0][0] = B; mat[0][1] = 1;
    mat[1][0] = E; mat[1][1] = 1;
    double vec[2];
    vec[0] = A + B * C;
    vec[1] = D + E * F;
    PDD res = calc(mat, vec);
    double x = res.first, y = res.second;
    assert(x==x); assert(y==y);
    double r = pow((x-x1)*(x-x1)+(y-y1)*(y-y1), 0.5);
    x = (double)(round(x*1000))/1000.0;
    y = (double)(round(y*1000))/1000.0;
    r = (double)(round(r*1000))/1000.0;    
    x = (x == -0.0) ? 0.0 : x;
    y = (y == -0.0) ? 0.0 : y;
    printf("%.3f %.3f %.3f\n", x, y, r);
  }
  return 0;
}

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

using namespace std;

int n;
double x[4],y[4],r;

int main(){
	cin >> n;
	while(n--){
		for(int i=0;i<3;i++){
			cin >> x[i] >> y[i];
		}
		x[3]=((y[2]-y[1])*(x[1]-x[0])*(x[2]-x[0])-(y[1]-y[0])*(x[1]+x[0])*(x[2]-x[0])+(y[2]-y[0])*(x[1]-x[0])*(x[2]+x[0]))/(2*(y[2]-y[0])*(x[1]-x[0])-(y[1]-y[0])*(x[2]-x[0]));
		y[3]=(y[0]+y[1])/2-(y[1]-y[0])*(x[3]-(x[0]+x[1])/2)/(x[1]-x[0]);
		printf("%.3lf %.3lf\n",x[3],y[3]);
	}
	return 0;
}