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 <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))));

		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;
	}
	/*
	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;
	}
	*/

	//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 <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;

const double EPS = 1e-3;

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

int main() {
  int n, p, q;
  double x[3], y[3], ta, tb, tc, sta, cta, r;
  double z[2], a, b, c, d, e, f;
  bool zf;
  cin >> n;
  for (int k=0;k<n;k++) {
    for (int i=0;i<3;i++) {
      cin >> x[i] >> y[i];
    }
    ta = dist(x[1], y[1], x[2], y[2]);
    tb = dist(x[0], y[0], x[1], y[1]);
    tc = dist(x[0], y[0], x[2], y[2]);
    cta = (ta*ta - tb*tb - tc*tc) / (2*tb*tc);
    sta = sqrt(1 - cta*cta);
    r = ta / sta / 2;

    double m[2][3] = { { 2*(x[0]-x[1]), 2*(y[0]-y[1]), x[0]*x[0]+y[0]*y[0]-x[1]*x[1]-y[1]*y[1] }, 
                       { 2*(x[0]-x[2]), 2*(y[0]-y[2]), x[0]*x[0]+y[0]*y[0]-x[2]*x[2]-y[2]*y[2] } };
    zf = false;
    for (int i=0;i<2;i++) {
      for (int j=0;j<2;j++) {
        if (m[i][j] == 0) {
          p = i; q = j;
          zf = true;
        }
      }
    }
    if (zf) {
      z[1-q] = m[p][2] / m[p][1-q];
      z[q] = (m[1-p][2] - m[1-p][1-q] * z[1-q]) / m[1-p][q];
    } else {
      a = m[0][0]; b = m[0][1]; c = m[0][2];
      d = m[1][0]; e = m[1][1]; f = m[1][2];
      z[1] = (f - d*c/a) / (e - d*b/a);
      z[0] = (c - b*z[1]) / a;
    }
    printf("%.3f %.3f %.3f\n", z[0], z[1], r);
  }
  return 0;
}

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

using namespace std;

int main(void){
	
	int n;
	double x[3],y[3],r,a,b,c,d[2],e[2],f[2],l,m;

	cin>>n;

	for(int i=0;i<n;i++){
		
		a=b=c=r=l=m=0;
		for(int i=0;i<2;i++){
			d[i]=e[i]=f[i]=0;
		}


		cin>>x[0]>>y[0]>>x[1]>>y[1]>>x[2]>>y[2];

		a=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));	//ツ各ツ陛督づ個陳キツつウ
		b=sqrt((x[2]-x[1])*(x[2]-x[1])+(y[2]-y[1])*(y[2]-y[1]));
		c=sqrt((x[0]-x[2])*(x[0]-x[2])+(y[0]-y[2])*(y[0]-y[2]));	
	
		r=a*b*c/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));			//ツ外ツ静堋円ツづ個板シツ径
		
		d[0]=2*(x[1]-x[0]);
		e[0]=2*(y[1]-y[0]);
		f[0]=x[0]*x[0]-x[1]*x[1]+y[0]*y[0]-y[1]*y[1];
		d[1]=2*(x[2]-x[0]);
		e[1]=2*(y[2]-y[0]);
		f[1]=x[0]*x[0]-x[2]*x[2]+y[0]*y[0]-y[2]*y[2];

		l=(e[0]*f[1]-e[1]*f[0])/(d[0]*e[1]-d[1]*e[0]);
		m=(f[0]*d[1]-f[1]*d[0])/(d[0]*e[1]-d[1]*e[0]);

		l=l*1000;
		l=l+0.5;
		l=int(l);
		l=l/1000;

		m=m*1000;
		m=m+0.5;
		m=int(m);
		m=m/1000;

		r=r*1000;
		r=r+0.5;
		r=(int)(r);
		r=r/1000;
		
		printf("%0.3lf %0.3lf %0.3lf\n",l,m,r);
	}
	return 0;
}