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>

int main()
{
	int n;
	std::cin >> n;
	for(int i = 0; i < n; ++i)
	{
		double x1,y1,x2,y2,x3,y3;
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		double const a = sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1));	
		double const b = sqrt((x3-x2)*(x3-x2) + (y3-y2)*(y3-y2));
		double const c = sqrt((x3-x1)*(x3-x1) + (y3-y1)*(y3-y1));

		double const cosA = (b*b+c*c-a*a)/(2*b*c);
		double const cosB = (a*a+c*c-b*b)/(2*a*c);
		double const cosC = (a*a+b*b-c*c)/(2*a*b);
		double const sinA = sqrt(1-cosA*cosA);

		double const r = a / (sinA * 2);
		//std::cout << "r = " << r << std::endl;

		//std::cout << "cosA = " << cosA << std::endl;
		//std::cout << "cosB = " << cosB << std::endl;
		//std::cout << "cosC = " << cosC << std::endl;

		if(-0.001 < cosA && cosA < 0.001) std::printf("%.3lf %.3lf", (x1+x2) / 2 , (y1+y2) / 2);
		else if (-0.001 < cosB && cosB < 0.001) std::printf("%.3lf %.3lf", (x2+x3) / 2 ,(y2+y3) / 2);
		else if (-0.001 < cosC && cosC < 0.001) std::printf("%.3lf %.3lf", (x1+x3) / 2 , (y1+y3) / 2);
		else
		{
			double const h = sqrt(r*r-(c/2)*(c/2));
				
			//std::cout << "c = " << c << std::endl;
			std::cout << "h = " << h << std::endl;
			
			double const xm = (x1+x3)/2;
			double const ym = (y1+y3)/2;
			std::cout << "xm = " << xm << " ym = " << ym << std::endl;

			double const sinD = (y1-y3) / sqrt((x3-x1)*(x3-x1) + (y3-y1)*(y3-y1));
			double const cosD = (x1-x3) / sqrt((x3-x1)*(x3-x1) + (y3-y1)*(y3-y1));
			std::cout << "sinD = " << sinD << " cosD = " << cosD << std::endl;

			double y4 = h * cosD;
			double x4 = h * sinD;
			std::cout << "x4 = " << x4 << " y4 = " << y4 << std::endl;

			if((sinD < 0 && cosD < 0) || (sinD > 0 && cosD < 0)) x4 *= -1;
			if((sinD == 0 && cosD < 0) || (sinD > 0 && cosD > 0) || (sinD < 0 && cosD > 0)) y4 *= -1;

			double const xd = xm + x4;
			double const yd = ym + y4;


			std::cout << xd << " " << yd;
		}
		std::printf(" %.3lf\n", r);
	}
	return 0;
}

C++: #include <cstdio>
#include <math.h>

int N;
double X1,Y1,X2,Y2,X3,Y3;

int main()
{
    scanf("%d",&N);
    while (N--)
    {
        scanf("%lf%lf%lf%lf%lf%lf",&X1,&Y1,&X2,&Y2,&X3,&Y3);
        double a = X1-X2;
        double b = Y1-Y2;
        double c = -(X1*X1+Y1*Y1-X2*X2-Y2*Y2);
        double d = X2-X3;
        double e = Y2-Y3;
        double f = -(X2*X2+Y2*Y2-X3*X3-Y3*Y3);
        double l = (c*e-b*f)/(a*e-b*d);
        double m = (d*c-a*f)/(d*b-e*a);
        double n = -(X1*X1+Y1*Y1+l*X1+m*Y1);
        printf("%.3f %.3f %.3f\n",l*-0.5,m*-0.5,sqrt(l*l+m*m-n*4.0)/2.0);
    }
}

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

double a,b,c,d,e,f,x,y,r;
int n;

double ifof(double a){
	int n = a*10000;
	if(n%10 >= 5)
		n+=10;
	else if(n%10 <= -5)
		n-=10;
	return n/10000.0;
}

double solvex(){
	double k,l;
	k = (c*c)-(a*a)+(d*d)-(b*b)-(2*y*(d-b));
	l = 2*(c-a);
	return k/l;
}

double solvey(){
	double k,l;
	k = ((e-a)*((c*c)-(a*a)+(d*d)-(b*b)))-((c-a)*((e*e)-(a*a)+(f*f)-(b*b)));
	l = 2*(((d-b)*(e-a))-((f-b)*(c-a)));
	return k/l;
}

int main(){
	std::cin >> n;
	for(int i=0; i<n; i++){
		std::cin >> a >> b >> c >> d >> e >> f;
		y = solvey();
		x = solvex();
		r = std::sqrt(((x-a)*(x-a)) + ((y-b)*(y-b)));
		std::printf("%.3f %.3f %.3f\n",ifof(x),ifof(y),ifof(r));
	}
	return 0;
}

C++: //04
#include<iostream>
#include<complex>
#include<iomanip>
#include<cmath>

#define S(X) ((X)*(X))

using namespace std;

int main(){
  int n;
  cin>>n;
  for(int i=0;i<n;i++){
    complex<double> p[3];
    for(int j=0;j<3;j++){
      double x,y;
      cin>>x>>y;
      p[j]=complex<double>(x,y);
    }
    double d[3];
    for(int j=0;j<3;j++){
      d[j]=abs(p[j]-p[(j+1)%3]);
    }
    double r=d[0]/2/sqrt(1-S((S(d[1])+S(d[2])-S(d[0]))/2/d[1]/d[2]));
    complex<double> c=p[0]+polar(r,arg(p[1]-p[0])+acos(d[1]/2/r));
    cout<<fixed<<setprecision(3)<<c.real()<<' '<<c.imag()<<' '<<r<<endl;
  }
  return 0;
}