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 <cstdio>
#include <bitset>
#include <cmath>
#include <complex>

#define EPS 1e-6
using namespace std;

double cross(complex<double> a, complex<double> b)
{
  return a.real() * b.imag() - a.imag() * b.real();
}

double dot(complex<double> a, complex<double> b)
{
  return a.real() * b.real() + a.imag() * b.imag();
}

int main()
{
  complex<double> coor[3];
  int N;
  scanf(" %d ", &N);
  for (int i = 0; i < N; i++) {
    for (int i = 0; i < 3; i++) {
      double x, y;
      scanf(" %lf %lf ", &x, &y);
      coor[i] = complex<double>(x, y);
      //printf("coor[%d](%.3lf, %.3lf)\n", i ,coor[i].real(), coor[i].imag());
    }
    complex<double> h[2], d[2];
    for (int j = 0; j < 2; j++) {
      h[j] = complex<double>((coor[j].real() + coor[j + 1].real()) / 2, (coor[j].imag() + coor[j + 1].imag()) / 2);
      d[j] = complex<double>(coor[j].real() - coor[j + 1].real(), coor[j].imag() - coor[j + 1].imag());
    }
    double D = cross(d[0], d[1]);
    complex<double>o(cross( complex<double>( dot( d[0], h[0] ) , dot( d[1], h[1] ) ), complex<double>( d[0].imag(), d[1].imag() ) ) / D,
					cross( complex<double>( d[0].real(), d[1].real() ), complex<double>( dot( d[0], h[0] ) , dot( d[1], h[1] ) ) ) / D);
    //printf("h1(x, y) = (%.3lf, %.3lf), h2(x, y) = (%.3lf, %.3lf), d1(x, y) = (%.3lf, %.3lf), d2(x, y) = (%.3lf, %.3lf), o(x, y) = ( %.3lf, %.3lf)\n", h[0].real(), h[0].imag(), h[1].real(), h[1].imag(), d[0].real(), d[0].imag(), d[1].real(), d[1].imag(), o.real(), o.imag());
    complex<double> t = o - coor[0];
    double r = abs(t);
    printf("%.3lf %.3lf %.3lf\n", o.real(), o.imag(), r);    
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
//#include <math.h>
//#include <string>
#include <cmath>
#include <iomanip>

int main();
double round(double dSrc, int iLen);

int main(){
	int cnt = 0;		//データセットの個数
	double x1, y1, x2, y2, x3, y3;
	std::vector<double> p;		//外接円の中心のx座標
	std::vector<double> q;		//外接円の中心のy座標
	std::vector<double> r;		//外接円の半径
	
	std::cin >> cnt;
	for (int i = 0; i < cnt; i++){
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		p.push_back(((y1 - y3) * (pow(y1, 2) - pow(y2, 2) + pow(x1, 2) - pow(x2, 2)) - (y1 - y2) * (pow(y1, 2) - pow(y3, 2) + pow(x1, 2) - pow(x3, 2))) / (2 * (y1 - y3) * (x1 - x2) - 2 * (y1 - y2) * (x1 - x3)));
		q.push_back(((x1 - x3) * (pow(x1, 2) - pow(x2, 2) + pow(y1, 2) - pow(y2, 2)) - (x1 - x2) * (pow(x1, 2) - pow(x3, 2) + pow(y1, 2) - pow(y3, 2))) / (2 * (x1 - x3) * (y1 - y2) - 2 * (x1 - x2) * (y1 - y3)));
		r.push_back(sqrt((pow((p[i] - x1), 2) + pow((q[i] - y1), 2))));
		
//#define AAA
#ifdef AAA
		int temp = p[i];
		if (p[i] - temp != 0){
			p[i] = round(p[i], 3);
		}

		temp = q[i];
		if (q[i] - temp != 0){
			q[i] = round(q[i], 3);
		}
		
		temp = r[i];
		if (r[i] - temp != 0){
			r[i] = round(r[i], 3);
		}

		std::cout << std::fixed << std::setprecision(3) << p[i] << " " << q[i] << " " << r[i] << std::endl;
	}
#endif
#ifndef	AAA
	for (int i = 0; i < cnt; i++){
		int temp = p[i];
		if (p[i] - temp != 0){
			p[i] = round(p[i], 3);
		}

		temp = q[i];
		if (q[i] - temp != 0){
			q[i] = round(q[i], 3);
		}
		
		temp = r[i];
		if (r[i] - temp != 0){
			r[i] = round(r[i], 3);
		}

		std::cout << std::fixed << std::setprecision(3) << p[i] << " " << q[i] << " " << r[i] << std::endl;
	}
#endif
	//while (1);
	return 0;
}

/**
* 指定桁で四捨五入を行う
* args:数値, 四捨五入する桁数
* return:結果
* 参考:http://oresi.hatenablog.com/entry/2014/06/23/【C%2CC%2B%2B】指定桁数で端数処理%EF%BC%88切り上げ、切り下げ
*/
double round(double dSrc, int iLen)
{
	double	dRet;

	dRet = dSrc * pow(10.0, iLen);
	dRet = (double)(int)(dRet + 0.5);

	return dRet * pow(10.0, -iLen);
}

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

/** Problem0010 : CIrcumscribed Circle of a Triangle **/
int main()
{
	int N;
	double X1, Y1, X2, Y2, X3, Y3;
	double a, b, c, d, e, f, l, m, n;
	cin >> N;
	for (int i=0; i<N; i++) {
		cin >> X1 >> Y1 >> X2 >> Y2 >> X3 >> Y3;
		a = X1-X2; b = Y1-Y2; c = -(X1*X1+Y1*Y1-X2*X2-Y2*Y2);
		d = X2-X3; e = Y2-Y3; f = -(X2*X2+Y2*Y2-X3*X3-Y3*Y3);
		l = (c*e - b*f)/(e*a - b*d); m = (c*d - a*f)/(b*d - a*e);
		n = -(X1*X1+Y1*Y1+l*X1+m*Y1);
		printf("%.3f %.3f %.3f\n",l*-0.5,m*-0.5,sqrt(l*l+m*m-n*4.0)/2.0);
	}
	return 0;
}