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 <iomanip>
#include <algorithm>
#include <math.h>
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;

		for ( int j = 0; j < 3; j++ )
		{
			swap( x1, x2 );
			swap( y1, y2 );
			swap( x2, x3 );
			swap( y2, y3 );

			double a = 2.0 * x1 - 2.0 * x2;
			double b = 2.0 * y1 - 2.0 * y2;
			double c = 2.0 * x2 - 2.0 * x3;
			double d = 2.0 * y2 - 2.0 * y3;

			if ( a * b - c * d == 0.0 )
				continue;

			double e = x1 * x1 - x2 * x2 + y1 * y1 + y2 * y2;
			double f = x2 * x2 - x3 * x3 + y2 * y2 + y3 * y3;

			double x0 = ( d * e - b * f ) / ( a * b - c * d );
			double y0 = ( a * f - c * e ) / ( a * b - c * d );

			double r1 = sqrt( (x1-x0)*(x1-x0) + (y1-y0)*(y1-y0) );
			double r2 = sqrt( (x2-x0)*(x2-x0) + (y2-y0)*(y2-y0) );
			double r3 = sqrt( (x3-x0)*(x3-x0) + (y3-y0)*(y3-y0) );

			cout.setf( ios::fixed, ios::floatfield );
			cout << setprecision(3);
			if ( r1 == r2 && r1 == r3 )
				cout << x0 << " " << y0 << " " << r1 << endl;
		}

	}
	return 0;
}

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

using namespace std;

int main()
{
    int n;
    double x1, y1, x2, y2, x3, y3;
    double a1, a2, b1, b2, c1, c2;
    double x, y, r;

    cin >> n;
    for (int i = 0; i < n; i++) {
        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;

        x = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
        y = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
        r = sqrt((x1 - x) * (x1 - x) + (y1 - y) * (y1 - y));

        cout << setprecision(3) << fixed;
        cout << x << " " << y << " " << r << endl;
    }

    return 0;
}

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

double EPS = 1e-10;
const double PI = acos(-1);

using namespace std;

double add(double a, double b){
  if(abs(a+b) < EPS * (abs(a)+abs(b)))return 0;
  return a+b;
}

struct point{
  double x, y;
  point(){}
  point(double x,double y) : x(x) , y(y){}

  point operator + (point p){
    return point(add(x,p.x), add(y,p.y));
  }

  point operator - (point p){
    return point(add(x,-p.x), add(y,-p.y));
  }

  point operator * (double d){
    return point(x*d,y*d);
  }

  point operator / (double d){
    return point(x/d,y/d);
  }
  
  bool operator == ( const point &p ) const {
    return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS;
  }
};

struct circle{
  point p;
  double r;
};

circle circumscribed_circle(point p, point q, point r){
  double a[2],b[2],c[2];
  circle res;
    
    a[0]=2*(q.x-p.x);
    b[0]=2*(q.y-p.y);
    c[0]=p.x*p.x-q.x*q.x+p.y*p.y-q.y*q.y;
    
    a[1]=2*(r.x-p.x);
    b[1]=2*(r.y-p.y);
    c[1]=p.x*p.x-r.x*r.x+p.y*p.y-r.y*r.y;
    
    res.p.x=(b[0]*c[1]-b[1]*c[0])/(a[0]*b[1]-a[1]*b[0]);
    res.p.y=(c[0]*a[1]-c[1]*a[0])/(a[0]*b[1]-a[1]*b[0]);
 
    res.r=sqrt(pow(fabs(p.x-res.p.x),2)+pow(fabs(p.y-res.p.y),2));
    
    return res;
}

int main(void) {
  
  int n;
  point in[3];
  circle ans;
 
  cin >> n;
  
  while(n--){
    for(int i=0;i<3;i++)
      cin >> in[i].x >> in[i].y;

    ans=circumscribed_circle(in[0], in[1], in[2]);

    printf("%.3f %.3f %.3f\n",ans.p.x,ans.p.y,ans.r);
  }
  
  return 0;
}

C++: #include <iostream>

using namespace std;

double dist(double x1, double y1, double x2, double y2){
	return sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
}

int main(){

	double x[3], y[3];
	int n;

	cin >> n;
	while(n-- > 0){
		for(int i = 0; i < 3; i++){
			cin >> x[i] >> y[i];
		}

		double a = x[1] - x[0];
		double b = y[1] - y[0];
		double c = x[2] - x[1];
		double d = y[2] - y[1];
		double e = a*a + b*b;
		double f = c*c + d*d;

		double p = (e*c - f*a) / (2 * (b*c - a*d));
		double q = (e*d - f*b) / (2 * (a*d - b*c));

		double r = dist(x[0], y[0], p, q);

		cout.setf(ios::fixed);
		cout.precision(4);
		cout << p << ' ' << q << ' ' << r << endl;
	}

	return 0;
}