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 <cassert>
#include <cmath>
#include <cstdio>

int main() {
  int n = 0;
  scanf("%d", &n);

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

    // x^2 + y^2 + lx + my + n = 0

    float A[3][3] = { {x1, y1, 1}, {x2, y2, 1}, {x3, y3, 1} };
    // detA = a00a11a22 + a10a21a02 + a20a01a12 - a00a21a12 - a20a11a02 - a10a01a22
    float detA =
        A[0][0] * A[1][1] * A[2][2]
      + A[1][0] * A[2][1] * A[0][2]
      + A[2][0] * A[0][1] * A[1][2]
      - A[0][0] * A[2][1] * A[1][2]
      - A[2][0] * A[1][1] * A[0][2]
      - A[1][0] * A[0][1] * A[2][2];

    assert(detA != 0);
    float invA[3][3] = {
        { (A[1][1] * A[2][2] - A[1][2] * A[2][1]) / detA,
          (A[0][2] * A[2][1] - A[0][1] * A[2][2]) / detA,
          (A[0][1] * A[1][2] - A[0][2] * A[1][1]) / detA },
        { (A[1][2] * A[2][0] - A[1][0] * A[2][2]) / detA,
          (A[0][0] * A[2][2] - A[0][2] * A[2][0]) / detA,
          (A[0][2] * A[1][0] - A[0][0] * A[1][2]) / detA },
        { (A[1][0] * A[2][1] - A[1][1] * A[2][0]) / detA,
          (A[0][1] * A[2][0] - A[0][0] * A[2][1]) / detA,
          (A[0][0] * A[1][1] - A[0][1] * A[1][0]) / detA }
    }; 

    float b[3] = {-(x1 * x1 + y1 * y1), -(x2 * x2 + y2 * y2), -(x3 * x3 + y3 * y3)};
    
    float l = invA[0][0] * b[0] + invA[0][1] * b[1] + invA[0][2] * b[2];
    float m = invA[1][0] * b[0] + invA[1][1] * b[1] + invA[1][2] * b[2];
    float n = invA[2][0] * b[0] + invA[2][1] * b[1] + invA[2][2] * b[2];
      
    // (x + l/2)^2 - l^2/4 + (y + m/2)^2 - m^2/4 + n = 0;
    // (x, y) = (-l/2, -m/2)
    // r = sqrt(l^2/4 + m^2/4 - n)
    float px = -l / 2;
    float py = -m / 2;
    float r = std::sqrt(l * l / 4.0 + m * m / 4.0 - n);
    
    printf("%.3f %.3f %.3f\n", px, py, r);
  }
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>
#include <memory.h>
#include <map>
#include <math.h>
#include <utility>


#pragma warning(disable:4996)
using namespace std;
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;
const double PI = 3.141592653589793238;
const Int INF = 999999999999;
typedef pair<int, int> P;


double A[2][2];
double C[2];


int main()
{
	int n;
	cin >> n;
	REP(i, n){
		double x1, x2, x3, y1, y2, y3;
		cin >> x1>> x2 >> x3 >> y1 >> y2 >> y3;

		double x21 = x2 - x1;
		double x31 = x3 - x1;
		double y21 = y2 - y1;
		double y31 = y3 - y1;

		double xc = ((y31*(x21*x21 + y21*y21)) - (y21*(x31*x31 + y31*y31))) /
			(2 * (x21*y31 - y21*x31));

		double yc = ((x21*(x31*x31 + y31*y31)) - (x31*(x21*x21 + y21*y21))) /
			(2 * (x21*y31 - y21*x31));	
		
		double d = (xc - x1)*(xc - x1) + (yc - y1)*(yc - y1);
		int xc_ = xc * 1000;
		int yc_ = yc * 1000;
		xc_ += (xc_ % 10) > 4 ? 10 : 0;
		yc_ += (yc_ % 10) > 4 ? 10 : 0;

		printf("%.3lf %.3lf %.3lf", xc, yc, sqrt(d));


	}

}

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

int main(){
	int n;
	double x1 , y1 , x2 , y2 , x3 , y3;
	double a1 , b1 , c1 , a2 , b2 , c2;
	double a , b , c;
	double px , py , r;
	double tmp;
	cin >> n;
	while(1){
		if(n==0) break;
		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);
		px=(b1*c2-b2*c1)/(a1*b2-a2*b1);
		py=(c1*a2-c2*a1)/(a1*b2-a2*b1);
		a=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
		b=sqrt((x2-x3)*(x2-x3)+(y2-y3)*(y2-y3));
		c=sqrt((x3-x1)*(x3-x1)+(y3-y1)*(y3-y1));
		r=a/(2*sqrt(1-((b*b+c*c-a*a)/(2*b*c))*((b*b+c*c-a*a)/(2*b*c))));
		tmp=px*1000+0.5;
		px=(int)tmp;
		px/=1000;
		tmp=px*1000+0.5;
		py=(int)tmp;
		py/=1000;
		tmp=r*1000+0.5;
		r=(int)tmp;
		r/=1000;
		cout << fixed << setprecision(3) << px << " " << py << " " << r << endl;
	}
	return 0;
}