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 <stdio.h>
#define _USE_MATH_DEFINES
#include <math.h>

int main(void){
	int n , i;
	float x1,x2,x3,y1,y2,y3,a1,a2,b1,b2,c1,c2,xp,yp,r;
	scanf("%d" , &n);
	for(i=1;i<=n;i++){
		scanf("%f %f %f %f %f %f" , &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 - y1);
		b2 = 2 * (y3 - y1);
		c2 = x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3;
		xp = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
		yp = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
		r = sqrt((xp - x1) * (xp - x1) + (yp - y1) * (yp - y1));
		printf("%.3f %.3f %.3f\n" , xp , yp , r);
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

struct TCoordinate
{
	double x;
	double y;
};

TCoordinate GetCenterPoint(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	double x1 = P1.x;
	double x2 = P2.x;
	double x3 = P3.x;
	double y1 = P1.y;
	double y2 = P2.y;
	double y3 = P3.y;
	double x, y, r;
	if(y1 == y2 || y2 == y3 || y3 == y1){
		if(y2 == y3){
			swap(x3, x1);
			swap(y3, y1);
		}else if(y3 == y1){
			swap(x2, x3);
			swap(y2, y3);
		}
		double xa = (x2 + x3) / 2;
		double ya = (y2 + y3) / 2;
		double a  = (-x3 + x2) / (y3 - y2);
		x = (x1 + x2) / 2;
		y = a * (x - xa) + ya;
	}else{
		double xa = (x1 + x2) / 2;
		double ya = (y1 + y2) / 2;
		double a  = (-x2 + x1) / (y2 - y1);
		double xb = (x2 + x3) / 2;
		double yb = (y2 + y3) / 2;
		double b  = (-x3 + x2) / (y3 - y2);
		x = (b * xb - a * xa - yb + ya) / (b - a);
		y = a * (x - xa) + ya;
	}
	return {x, y};
}

double GetRadius(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	double x1 = P1.x;
	double x2 = P2.x;
	double x3 = P3.x;
	double y1 = P1.y;
	double y2 = P2.y;
	double y3 = P3.y;
	double x, y, r;
	if(y1 == y2 || y2 == y3 || y3 == y1){
		if(y2 == y3){
			swap(x3, x1);
			swap(y3, y1);
		}else if(y3 == y1){
			swap(x2, x3);
			swap(y2, y3);
		}
		double xa = (x2 + x3) / 2;
		double ya = (y2 + y3) / 2;
		double a  = (-x3 + x2) / (y3 - y2);
		x = (x1 + x2) / 2;
		y = a * (x - xa) + ya;
	}else{
		double xa = (x1 + x2) / 2;
		double ya = (y1 + y2) / 2;
		double a  = (-x2 + x1) / (y2 - y1);
		double xb = (x2 + x3) / 2;
		double yb = (y2 + y3) / 2;
		double b  = (-x3 + x2) / (y3 - y2);
		x = (b * xb - a * xa - yb + ya) / (b - a);
		y = a * (x - xa) + ya;
	}
	return sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1));
}

int main()
{
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		TCoordinate P1, P2, P3;
		cin >> P1.x >> P1.y >> P2.x >> P2.y >> P3.x >> P3.y;
		TCoordinate CenterPoint = GetCenterPoint(P1, P2, P3);
		double Radius = GetRadius(P1, P2, P3);
		cout << fixed << setprecision(3);
		cout << CenterPoint.x << ' ' << CenterPoint.y << ' ' << Radius << endl;
	}
	return 0;
}

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

using namespace std;

int main(void){
	int n, i, j;
	double x1,y1,x2,y2,x3,y3, r, px, py;
	scanf("%d",&n);
	for(i = 0;i < n;i++){
		scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
		if((y2 - y1) * (x3 - x1) - (y3 - y1) * (x2 - x1) == 0){
			if(x1 == x2 && x2 == x3){
				px = x1;
				py = (max(y1,max(y2,y3)) + min(y1,min(y2,y3))) / 2;
				r = fabs(max(y1,max(y2,y3)) - py);
			}
			else{
				double tx1, ty1, tx2, ty2;
				if(x1 > x2 && x1 > x3){
					tx1 = x1, ty1 = y1;
				}else if(x2 > x1 && x2 > x3){
					tx1 = x2, ty1 = y2;
				}else{
					tx1 = x3, ty1 = y3;
				}
				if(x1 < x2 && x1 < x3){
					tx2 = x1, ty2 = y1;
				}else if(x2 < x1 && x2 < x3){
					tx2 = x2, ty2 = y2;
				}else{
					tx2 = x3, ty2 = y3;
				}
				px = (tx1 + tx2) / 2;
				py = (ty1 + ty2) / 2;
				r = sqrt(pow(tx1 - px,2.0) + pow(ty1 - py,2.0));
			}
		}else{
			py = ((x3 - x1) * (pow(x2,2.0) - pow(x1,2.0) + pow(y2,2.0) - pow(y1,2.0)) - (x2 - x1) * (pow(x3,2.0) - pow(x1,2.0) + pow(y3,2.0) - pow(y1,2.0))) / (2 * ((y2 - y1) * (x3 - x1) - (y3 - y1) * (x2 - x1)));
			px = ((y3 - y1) * (pow(y2,2.0) - pow(y1,2.0) + pow(x2,2.0) - pow(x1,2.0)) - (y2 - y1) * (pow(y3,2.0) - pow(y1,2.0) + pow(x3,2.0) - pow(x1,2.0))) / (2 * ((x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)));
			r = sqrt(pow(px - x1,2.0) + pow(py - y1,2.0));
		}
		printf("%f %f %f\n",px,py,r);
	}
	return 0;
}