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 <complex>
#include <cmath>

using namespace std;

typedef complex<double> Point;

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

double r_(double x) {
    if(abs(x) < 0.001) return 0;
    return x;
}

int main() {
    int N;

    cin >> N;
    while(N--) {
        double x1, x2, x3, y1, y2, y3;
        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);

        //pc1 + k*ph1 == pc2 + l*ph2
        //pc1-pc2 == l*ph2 - k*ph1
        //d = pc1-pc2
        //l*ph2.x - k*ph1.x == d.x
        //l*ph2.y - k*ph1.y == d.y
        Point d = pc1-pc2;
        double det = ph2.real()*-ph1.imag() + ph1.real()*ph2.imag();
        double l = (d.real()*-ph1.imag() - d.imag()*ph2.imag()) / det;
        double k = (d.real()*ph1.real() + d.imag()*ph2.real()) / det;
        Point center = pc1 + ph1 * k;
        R = abs(center-p1);

        printf("%.3f %.3f %.3f\n", r_(center.real()), r_(center.imag()), r_(R));
    }
    return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

double distance(double x1, double y1, double x2, double y2){
    return sqrt(pow(abs(x1-x2), 2)+pow(abs(y1-y2), 2));
}

int main(){
    int q; cin>> q;
    for(int z=0; z < q; z++){
        double x1,y1,x2,y2,x3,y3;
        cin >> x1 >>y1 >>x2 >>y2 >>x3 >>y3;
        double a = distance(x1, y1, x2, y2), b = distance(x2, y2, x3, y3), c = distance(x3, y3, x1, y1);
        double s = (a+b+c)/2;
        double r = (a*b*c)/(4.*sqrt(s*(s-a)*(s-b)*(s-c)));
        double a1 = 2*(x2-x1), b1 = 2*(y2-y1), c1 = x1*x1-x2*x2+y1*y1-y2*y2;
        double a2 = 2*(x3-x1), b2 = 2*(y3-y1), 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);
        printf("%.4f %.4f %.4f\n", x, y, r);
    }
}

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

int main()
{
	double x1, y1, x2, y2, x3, y3;
	double a1, b1, c1, a2, b2, c2;
	double xp, yp, r;
	int loop;
	
	cin >> loop;
	
	while(loop>0){
		while(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;
		
			xp = (b1*c2-b2*c1)/(a1*b2-a2*b1);
			yp = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		
			r = sqrt(pow((x1-xp), 2) + pow((y1-yp), 2));
				
			cout << fixed << setprecision(3) << xp << " " << yp << " " << r << endl;
			
			--loop;
			if(loop==0){
				break;
			}
		}
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>
using namespace std;
long double ff(long double a){
  
  a*=10000.0;
  //  cout<<a<<endl;
  long long c = a;
  if(c%10>4)c+=10;
  c/=10;
  return (long double)c/1000.0;
}



long double ax,ay,bx,by,cx,cy,ox,oy,dx,dy,ex,ey;
//ack はACの傾き  eos はEOの切片
long double abk,ack,dok,eok,eos,dos,eps=0.0;
int n;
int main(){

  cin>>n;
  for(int i=0;i<n;i++){
    cin>>ax>>ay>>bx>>by>>cx>>cy;
    //ABの中点をD,  ACの中点をE 外接円の中心をO とする
    dx=(ax+bx)/2.0;
    dy=(ay+by)/2.0;
    ex=(ax+cx)/2.0;
    ey=(ay+cy)/2.0;
    
    if(ax==bx){
      abk=(ay-by)/0.000000000001;
    }else{
      abk=(ay-by)/(ax-bx);
    }
    if(abk==0.0)abk=0.000000000001;
    dok=-1.0/abk;
    dos=dy-dx*dok;
    
    
    if(ax==cx){
      ack=(ay-cy)/0.000000000001;
    }else{
      ack=(ay-cy)/(ax-cx);
    }
    if(ack==0.0)ack=0.000000000001;
    eok=-1.0/ack;
    eos=ey-ex*eok;
    
    //Y  =  dok X  + dos
    //Y  =  eok X  + eos
    //dok X + dos = eok X + eos
    
    long double Y,X;
    if(dok-eok==0.0){
      X=(eos-dos)/0.000000000001;
    }else{
      X=(eos-dos)/(dok-eok);
    }
    Y= dok * X + dos;
    
    printf("%.3Lf %.3Lf ",X,Y);
    //cout<<ax<<' '<<X<<' '<<ay<<' '<<Y<<endl;
    printf("%.3Lf\n" ,  sqrt( pow(ax-X,2) + pow(ay-Y,2) ));

    
    
  }
  return 0;
}