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 <cstdio>
#include <cmath>
using namespace std;

/*
 * ax+by=c, dx+ey=f ツづーツ嘉ーツつュツ。
 * ツ姪淞づィツ値ツつェ0ツづ按づァツ青ャツ古キツ、1ツづ個湘ェツ債づ溝e-bdツつェEPSツ暗按可コツづ可づ按づづつ「ツづ古ォツ債キツ妥・ツつォツつ「ツ。
 */
int solve_linear_equations( double a, double b, double c, 
  double d, double e, double f, double& x, double& y )
{
  double EPS = 1e-9;
  double delta = a*e-b*d;
  x = 0.0;
  y = 0.0;
  if( (delta < 0 && -delta < EPS) || (delta >=0 && delta < EPS) ) return 1;
  x = ((c*e-b*f) / delta) + 0.0;
  y = ((a*f-c*d) / delta) + 0.0;
  return 0;
}

int main( void )
{
  int n;
  double x1,y1,x2,y2,x3,y3;
  cin >> n;
  for( int i=0; i<n; i++ ){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double a,b,c,d,e,f,x,y,r;
    a = 2*(x1-x2);
    b = 2*(y1-y2);
    c = x1*x1+y1*y1-x2*x2-y2*y2;
    d = 2*(x1-x3);
    e = 2*(y1-y3);
    f = x1*x1+y1*y1-x3*x3-y3*y3;
    solve_linear_equations( a,b,c,d,e,f,x,y );
    r = sqrt(x1*x1-2*x1*x+x*x+y1*y1-2*y1*y+y*y);
    printf( "%.3lf %.3lf %.3lf\n",x,y,r );
  }
  return 0;
}

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

int main(){

  double x[3], y[3], mx, my, lx, ly, dm, dl, cx, cy, r;
  
  for(int i=0; i<3; i++){
    cin >> x[i] >> y[i];
  }
  
  mx = (x1 + x2) / 2;
  my = (y1 + y2) / 2;
  dm = (y1 - y2) / (x1 - y2);
  lx = (x2 + x3) / 2;
  ly = (y2 + y3) / 2;
  dl = (y2 - y3) / (x2 - x3);
  cx = (my + dm * mx) - (ly + dl * lx);
  cy = (ly + dl * lx) * dm - (my + dm * mx) * dl;
  
  r = sqrt((x1 - cx) * (x1 - cx) + (y1 - cy) * (y1 - cy));

  cx = round(cx * 1000) / 1000;
  cy = round(cy * 1000) / 1000;
  r = round(r * 1000) / 1000;

  printf("%.3d %.3d %.3d\n", cx, cy, r);

  return 0;
}

C++: #include<stdio.h>
#include<math.h>
int main(){
    int a;
    scanf("%d",&a);
    while(a--){
        double x1,y1,x2,y2,x3,y3;
        scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
        double a1=2*(x2-x1);
        double b1=2*(y2-y1);
        double c1=x1*x1-x2*x2+y1*y1-y2*y2;
        double a2=2*(x3-x1);
        double b2=2*(y3-y1);
        double c2=x1*x1-x3*x3+y1*y1-y3*y3;
        double x=(b1*c2-b2*c1)/(a1*b2-a2*b1);
        double y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
        double r=hypot(x-x1,y-y1);
        printf("%.3lf %.3lf %.3lf\n",x,y,r);
    }
}

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

const double EPS = 1E-8;
typedef vector<double> vd;
typedef vector<vd> mat;

// 拡大係数行列で解く
vector<double> GaussJordan(const mat& A, const vd b) {
  int n = A.size();
  mat B(n, vd(n+1));
  for(int i=0; i<n; i++) {
    for(int j=0; j<n; j++) B[i][j] = A[i][j];
  }
  for(int i=0; i<n; i++) {
    B[i][n] = b[i];
  }

  for(int i=0; i<n; i++) {
    int pivot = i;
    for(int j=i; j<n; j++) {
      if(abs(B[j][i]) > abs(B[pivot][i])) pivot = j;
    }
    swap(B[i], B[pivot]);

    if(abs(B[i][i]) < EPS) return vd();
    
    for(int j=i+1; j<=n; j++) B[i][j] /=B[i][i];
    for(int j=0; j<n; j++) {
      if(i != j) {
	for(int k=i+1; k<=n; k++) B[j][k] -= B[j][i] * B[i][k];
      }
    }
  }
  vd x(n);
  for(int i=0; i<n; i++) x[i] = B[i][n];
  return x;
}

int main() {
  int tcase;
  cin >> tcase;
  while(tcase--) {
    double x[3], y[3];
    cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
    
    mat A(3, vd(3, 0));
    for(int i=0; i<3; i++) {
      for(int j=0; j<3; j++) {
	if(j == 2) A[i][j] = 1;
	else A[i][j] = (j % 2 ? y[i] : x[i]);
      }
    }
    
    vd b(3);
    for(int i=0; i<3; i++) b[i] = -(x[i]*x[i] + y[i]*y[i]);

    vd v = GaussJordan(A, b);
    
    double cX = -v[0]/2, cY = -v[1]/2;
    double r = sqrt(pow(x[0]-cX, 2) + pow(y[0]-cY, 2));
    printf("%.3f %.3f %.3f\n", cX, cY, r);
  }
  
  return 0;
}

C++: #include <iostream>
 #include <math.h>
 #include <string>
using namespace std;

int main() {
	int n;
	cin>>n;
	for(int h=0;h<n;h++){
double x1,y1,x2,y2,x3,y3,x2m,x3m,y2m,y3m,x21,y21,x31,y31,xy21,xans,yans;
cin>>x1>>y1>>x2>>y2>>x3>>y3;
	x2m=(x1+x2)/2;
	y2m=(y1+y2)/2;
	x3m=(x1+x3)/2;
	y3m=(y1+y3)/2;
	x21=x2-x1;
	x31=x3-x1;
	y21=y2-y1;
	y31=y3-y1;
	
	
if(y1!=y2){
	xy21=x21/y21;
	xans=(x31*x3m-y31*(xy21*x2m+y3m-y2m))/(x31-y31*xy21);
	yans=-xy21*(xans-x2m)+y2m;
}	
else{
	xans=x2m;
	yans=(x3m-xans)*x31/y31+y3m;
}
double r;
r=sqrt((xans-x1)*(xans-x1)+(yans-y1)*(yans-y1));
double x=xans,y=yans;
	printf("%.3f %.3f %.3f\n",xans,yans,r);
}

	return 0;
}