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;
  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))/2;


    double x = A*(B+C-A) / s;
    double y = B*(C+A-B) / s;

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

  }




  return 0;
}

C++: #include <bits/stdc++.h>
#include <vector>
using namespace std;

double n, r, x1, y1, x2, y2, x3, y3, xa, ya, xb, yb, xc, yc, ma, mb, mc, y_hasil, x_hasil;

int main(){
	cin >> n;
	for(int i = 1; i <= n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		ma = (x2-x1)/(y1-y2);
		mb = (x3-x2)/(y2-y3);
		mc = (x3-x1)/(y1-y3);
		xa = (x1+x2)/2;
		ya = (y1+y2)/2;
		xb = (x2+x3)/2;
		yb = (y2+y3)/2;
		xc = (x3+x1)/2;
		yc = (y3+y1)/2;
		
	//	cout << ma << " " << xa << " " << mb << " " << xb << " " << yb << " " << ya << "\n";
		if(y1 - y2 == 0){
			x_hasil = (x2 + x1) / 2;
			y_hasil = mb*(x_hasil - xb) + yb;
		}else if(y2 - y3 == 0){
			x_hasil = (x2 + x3) / 2;
			y_hasil = ma*(x_hasil - xa) + ya;
		}else{
			x_hasil = (ma*xa - mb*xb + yb - ya)/(ma - mb);
			y_hasil = ma*(x_hasil - xa) + ya;
		}
		
		r = sqrt((x_hasil - x1)*(x_hasil - x1) + (y_hasil - y1)*(y_hasil - y1));
		
		cout << fixed << setprecision(3) << x_hasil << " " << y_hasil << " " << r << "\n";
	}
	return 0;
}

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

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int n;
	cin >> n;
	
	while(n-- > 0){
	
	double x1, y1, x2, y2, x3, y3;
	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
	
	double a = 2 * (x2 - x1);
	double b = 2 * (y2 - y1);
	double c = x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2;
	double d = 2 * (x3 - x2);
	double e = 2 * (y3 - y2);
	double f = x2 * x2 - x3 * x3 + y2 * y2 - y3 * y3;
		
	double cx = (b * f - c * e)/(a * e - d * b);
	double cy = (a * f - c * d)/(b * d - a * e);
		
	double r = sqrt((x1 - cx)*(x1 - cx) + (y1 - cy)*(y1 - cy));
	
	cout << fixed << setprecision(3) << cx << " " << cy << " " << r << endl;
	
	}
	
	return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    double p[3][2],m[2][2],a[2],inv[2][2],o[2],det;
    for(int i=0; i<3; i++) for(int j=0; j<2; j++) cin >>p[i][j];
    for(int i=1; i<3; i++) for(int j=0; j<2; j++) p[i][j]-=p[0][j];
    for(int i=0; i<2; i++) for(int j=0; j<2; j++) m[i][j] = 2*p[i+1][j];
    for(int i=0; i<2; i++) a[i] = p[i+1][0]*p[i+1][0]+p[i+1][1]*p[i+1][1];
    det = m[0][0]*m[1][1]-m[0][1]*m[1][0];
    inv[0][0] = m[1][1]/det;
    inv[0][1] = -m[0][1]/det;
    inv[1][0] = -m[1][0]/det;
    inv[1][1] = m[0][0]/det;
    for(int i=0; i<2; i++) o[i] = inv[i][0]*a[0]+inv[i][1]*a[1];
    printf("%.3lf %.3lf %.3lf\n",o[0]+p[0][0],o[1]+p[0][1],sqrt(o[0]*o[0]+o[1]*o[1]));
  }
  return 0;
}