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<math.h>
#include<stdio.h>
using namespace std;
int main() {
    double x1, x2, x3, px, y1, y2, y3, py,r;
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        if (y1 == y2) {
            px = (x1 + x2) / 2;
            py = (x1 - x3)*(x2 - x3) / (2 * (y3 - y1)) + (y1 + y3) / 2;
        }
        else if (y1 == y3) {
            px = (x1 + x3) / 2;
            py = (x1 - x2)*(x3 - x2) / (2 * (y2 - y1)) + (y1 + y2) / 2;
        }
        else {
            px = ((x1*x1 - x2*x2) / (y2 - y1) - (x1*x1 - x3*x3) / (y3 - y1) + y3 - y2) / (2 * ((x1 - x2) / (y2 - y1) - (x1 - x3) / (y3 - y1)));
            py = (px - (x1 + x2) / 2)*(x1 - x2) / (y2 - y1) + (y1 + y2) / 2;
        }
        r = sqrt((px - x1)*(px - x1) + (py - y1)*(py - y1));
        printf("%.3f %.3f %.3f\n", px, py, r);
    }
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
int main(){
  double a[2],b[2],c[2];
  int n;
  cin >>n;
  while(n--){
    cin>>a[0]>>a[1]>>b[0]>>b[1]>>c[0]>>c[1];
    b[0]-=a[0];
    b[1]-=a[1];
    c[0]-=a[0];
    c[1]-=a[1];
    double m[2][2],ans[2],rev[2][2];
    m[0][0] = 2*b[0];
    m[0][1] = 2*b[1];
    ans[0] = b[0]*b[0]+b[1]*b[1];
    m[1][0] = 2*c[0];
    m[1][1] = 2*c[1];
    ans[1] = c[0]*c[0]+c[1]*c[1];
    double det = m[0][0]*m[1][1]-m[1][0]*m[0][1];
    rev[0][0] = m[1][1]/det;
    rev[1][0] = -m[1][0]/det;
    rev[0][1] = -m[0][1]/det;
    rev[1][1] = m[0][0]/det;
    double x = ans[0]*rev[0][0]+ans[1]*rev[0][1];
    double y = ans[0]*rev[1][0]+ans[1]*rev[1][1];
    double r = sqrt(x*x+y*y);
    printf("%.03lf %.03lf %.03lf\n",x+a[0],y+a[1],r);
  }
  return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>

using namespace std;

int main()
{
	int i;
	cin >> i;
	while (i--)
	{
		double x1, x2, x3, y1, y2, y3;
		double X1, X2, Y1, Y2;
		double x0, y0, R1, R2, R3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		X1 = x2 - x1; X1 = X1 / 2;
		Y1 = y2 - y1; Y1 = Y1 / 2;
		X2 = x3 - x1; X2 = X2 / 2;
		Y2 = y3 - y1; Y2 = Y2 / 2;

		y0 = (X1*X2*X2 - X2*X1*X1 + X1*Y2*Y2 - X2*Y1*Y1) / (X1*Y2 - X2*Y1) + y1;
		x0 = (Y1*Y2*Y2 - Y2*Y1*Y1 + Y1*X2*X2 - Y2*X1*X1) / (Y1*X2 - Y2*X1) + x1;

		R1 = sqrt((x0 - x1)*(x0 - x1) + (y0 - y1)*(y0 - y1));

		printf("%.3lf %.3lf %.3lf", x0, y0, R1);
	}
	system("pause");
	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);
}