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 <iomanip>
using namespace std;


int main(){
  int n;
  double a,b,c,d,e,f;
  double x1,x2,x3,y1,y2,y3;
  double px,py;
  double r;

  cin >> n;

  for(int i=0;i<n;i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    a = (x2-x1)*2.0;
    b = (y2-y1)*2.0;
    c = (x2*x2 + y2*y2 - x1*x1 - y1*y1);
    d = (x3-x1)*2.0;
    e = (y3-y1)*2.0;
    f = (x3*x3 + y3*y3 - x1*x1 - y1*y1);
    px = (c*e-b*f)/(a*e-b*d);
    py = (a*f-c*d)/(a*e-b*d);
    r = sqrt((px-x1)*(px-x1) + (py-y1)*(py-y1));
    //printf("%.3f %.3f %.3f\n",px,py,r);
    cout << fixed << setprecision(3) << px << " " << py << " " << r << endl;



  }

  return 0;
}

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

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

void solve(){
  float tmp1 = f-(c*d/a);
  float tmp2 = e-(b*d/a);
  y = tmp1/tmp2;
  x = (c - b * y) / a;
  printf("%.3f %.3f ", x, y);        
  //printf("y=%f\n", (f-(c*d/a))/(e-(b*d/a)));
  // ツ四ツ偲個古慊禿シ
  int tx = ((int)(10000 * x)) % 10;
  if (tx >= 5) {
    x += 0.001;
  }
  int ty = ((int)(10000 * y)) % 10;
  if (ty >= 5) {
    y += 0.001;
  }
}

int main(int argc, char const* argv[]) {
  int n;
  cin>>n;
  for (int i = 0; i < n; i += 1) {
    float 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();
    float r = (x - x1)*(x - x1) + (y - y1)*(y - y1);
    int tr = ((int)(10000 * r)) % 10;
    if (tr >= 5) {
      r += 0.001;
    }
    printf("%.3f\n", sqrt(r));
  }  
  return 0;
}

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

using namespace std;

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

	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double x = ((y1-y3)*(y1*y1-y2*y2+x1*x1-x2*x2)-(y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3))/(2*(y1-y3)*(x1-x2)-2*(y1-y2)*(x1-x3));
		double y = ((x1-x3)*(x1*x1-x2*x2+y1*y1-y2*y2)-(x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3))/(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));

		printf("%.3f %.3f %.3f\n", x, y, sqrt(pow((x1 - x), 2) + pow((y1 - y), 2)));
	}
	return 0;
}

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

using namespace std;

typedef complex<double> Point;

bool eqv(double d1, double d2) {
    return abs(d1-d2) < 1e-4;
}

int main() {
    int N;
    double x1, x2, x3, y1, y2, y3;

    while(true) {
        cin >> N;
        if(!N) break;

        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        Point p1(x1, y1), p2(x2, y2), p3(x3, y3);

        double R = abs(abs(p3-p2) / sin(arg(p2-p1) - arg(p3-p1))) / 2;
        Point pc1 = (p1+p2) / 2.;
        Point ph1 = (pc1-p1) * Point(0, 1);
        Point pc2 = (p1+p3) / 2.;
        Point ph2 = (pc2-p1) * Point(0, 1);

        Point d = pc2-pc1;
        double ratio = ph2.real() / ph2.imag();
        double a = (d.real()-d.imag()*ratio) / (ph1.real()-ph1.imag()*ratio);
        Point center = pc1 + ph1 * a / abs(ph1);

        int i1 = (int)center.real(), d1 = ((int)((center.real()-i1)*10000)+5)/10;
        int i2 = (int)center.imag(), d2 = ((int)((center.imag()-i2)*10000)+5)/10;
        int i3 = (int)R, d3 = ((int)((R-i3)*10000)+5)/10;
        printf("%d.%03d %d.%03d %d.%03d\n", i1, d1, i2, d2, i3, d3);
    }
    return 0;
}

C++: #include<complex>
#include<cstdio>
#include<cmath>
using namespace std;
typedef complex<double> P;
double F(double a,double b,double c){
	return acos((b*b+c*c-a*a)/(2*b*c));
}
int main(){
	int n;
	double d,e,f,g,h,i,x,y,r;
	scanf("%d",&n);
	for(;n--;){
		scanf("%lf%lf%lf%lf%lf%lf",&d,&g,&e,&h,&f,&i);
		P a(d,g),b(e,h),c(f,i);
		double A=abs(b-c),B=abs(c-a),C=abs(a-b),s=F(A,B,C),t=F(B,C,A),u=F(C,A,B);
		r = A/sin(s)/2.;
		x = ( d*sin(2*s) + e*sin(2*t) + f*sin(2*u) ) /
			( sin(2*s) + sin(2*t) + sin(2*u) );
		y = ( g*sin(2*s) + h*sin(2*t) + i*sin(2*u) ) /
			( sin(2*s) + sin(2*t) + sin(2*u) );
		printf("%.3f %.3f %.3f\n",x,y,r);
	}
}