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++: 
// Circumscribed Circle of a Triangle


#include<stdio.h>
#include<iostream>
#include<math.h>
#include<iomanip>
using namespace std;

int main()
{
	int n;
	cin >> n;
	double x[3];
	double y[3];
	double r;

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

	 double a = 2*(x[1]-x[0]);
	 double b = 2*(y[1]-y[0]);
	 double c = 2*(x[2]-x[0]);
	 double d = 2*(y[2]-y[0]);

	 double e = pow(x[1],2.0)-pow(x[0],2.0)+pow(y[1],2.0)-pow(y[0],2.0);
	 double f = pow(x[2],2.0)-pow(x[0],2.0)+pow(y[2],2.0)-pow(y[0],2.0);

	 if(a==0){
	 	swap(a,c);
	 	swap(b,d);
	 	swap(e,f);
	 }

	b = b/a;
	e = e/a;
	a = a/a;
	 
	 
	d = d-b*c;
	f = f-e*c;
	c = c-a*c;

	f = f/d;
	d = d/d;

	b = b-d*b;
	e = e-f*b;

	r = sqrt(pow((e-x[0]),2.0)+pow((f-y[0]),2.0));
	
	cout << fixed;
	cout << setprecision(3) << e <<" "<< f << " " << r << endl;
	}
	return 0;

}

C++: #include<bits/stdc++.h>
using namespace std;
int main()
{
    int t;
    cin >> t;
    while(t--)
    {
        double x1,x2,x3,y1,y2,y3,a,b,c,r,d,p1,p2;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        c = sqrt(pow(x2-x1,2)+pow(y2-y1,2));
        b = sqrt(pow(x3-x1,2)+pow(y3-y1,2));
        a = sqrt(pow(x3-x2,2)+pow(y3-y2,2));
        r = (a*b*c) / (sqrt((a+b+c)*(b+c-a)*(c+a-b)*(a+b-c)));
        d = 2*(x1*(y2-y3) + x2*(y3-y1) + x3*(y1-y2));
        p1 =((pow(x1,2)+pow(y1,2))*(y2-y3)+(pow(x2,2)+pow(y2,2))*(y3-y1)+(pow(x3,2)+pow(y3,2))*(y1-y2))/d;
        p2 =((pow(x1,2)+pow(y1,2))*(x3-x2)+(pow(x2,2)+pow(y2,2))*(x1-x3)+(pow(x3,2)+pow(y3,2))*(x2-x1))/d;
        cout << fixed << setprecision(3) << p1 <<" "<< p2 <<" "<< r << endl;
    }
}

C++: import java.util.Scanner;
import java.lang.Math;

class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		int n = scan.nextInt();
		for(int i = 0; i < n; i++) {
			double x1,y1,x2,y2,x3,y3;
			
			x1 = scan.nextDouble();
			y1 = scan.nextDouble();
			x2 = scan.nextDouble();
			y2 = scan.nextDouble();
			x3 = scan.nextDouble();
			y3 = scan.nextDouble();
			
			double a1, a2, b1, b2, c1, c2, px, py;

			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;
			
			double temp = a1*b2 - a2*b1;
			
			px = (b1*c2 - b2*c1) / temp;
			py = (c1*a2 - c2*a1) / temp;
			
			double r = Math.sqrt((x1-px)*(x1-px) + (y1-py)*(y1-py));
			
			System.out.printf("%.3f %.3f %.3f", px, py, r);
		}
	}
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
 struct vect{
    double x,y;
 };
int main(){
  vect a,b,c,p,l1,l2,M1,M2;
  double t1,r;
  int n;
  cin >> n;
  for(int i = 0;i < n;i++){
    cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;
    M1.x = (a.x + b.x) /2;
    M1.y = (a.y + b.y) /2;
    M2.x = (b.x + c.x)/2;
    M2.y = (b.y + c.y)/2;
    l1.y = -(b.x - a.x);
    l1.x = b.y -a.y;
    l2.y = -(c.x-b.x);
    l2.x = c.y -b.y;
    t1 = 1/(-l1.x * l2.y + l2.x * l1.y) *( -l2.y * (M2.x -M1.x) + l2.x * (M2.y -M1.y));
    p.x = M1.x + l1.x * t1;
    p.y = M1.y + l1.y * t1;
    r = sqrt((p.x - a.x)*(p.x - a.x) +(p.y - a.y)*(p.y -a.y));
    printf("%.3f %.3f %.3f\n",p.x,p.y,r);
  }
}

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

using namespace std;

#define EPS 1e-9

struct P {
	double x, y;
};

double abs(P p) {
	return sqrt(p.x * p.x + p.y * p.y);
}
double dot(P p1, P p2) {
	return p1.x * p2.x + p1.y * p2.y;
}
double xx(double r) {
	return r*r;
}

int main() {
	int n; cin >> n;
	for (int t = 0; t < n; ++t) {
		double x1, x2, x3, y1, y2, y3;
		cin >> x1 >> y1;
		cin >> x2 >> y2;
		cin >> x3 >> y3;

		P a = (P){ x2 - x1, y2 - y1 };
		P b = (P){ x3 - x2, y3 - y2 };
		P c = (P){ x1 - x3, y1 - y3 };

		double R =  abs(a) / (sqrt(1 - xx(dot(b, c) / abs(b) / abs(c))) * 2);
		double l = sqrt( R*R - xx(abs(c)/2) );
		double gx = (x1+x3)/2, gy = (y1+y3)/2;
		P e = (P){ -c.y/abs(c), c.x/abs(c) };
		P eg = (P){ e.x * l, e.y * l };
		P g1 = (P){ gx + eg.x, gy + eg.y };
		P g2 = (P){ gx - eg.x, gy - eg.y };
		if ( abs(xx(x1-g1.x) + xx(y1-g1.y) - R*R) < EPS ) gx = g1.x, gy = g1.y;
		else gx = g2.x, gy = g2.y;
		if ( abs(gx) < EPS ) gx = abs(gx);
		if ( abs(gy) < EPS ) gy = abs(gy);
		printf("%.3f %.3f %.3f\n", gx, gy, R);
	}
}