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 <iostream>
#include <map>
#include <queue>
#include <functional>
#include <complex>
#include <cmath>
using namespace std;
#define P complex<double>
#define X real()
#define Y imag()
#define EPS 1e-10

class Triangle {
	private:
		P a, b, c;
		double edgeA, edgeB, edgeC;
		double angleA, angleB, angleC;
		double getAngle(double e1, double e2, double e3) {
			return acos((e2 * e2 + e3 * e3 - e1 * e1) / (2 * e2 * e3));
		}
	public:
		Triangle(P p1, P p2, P p3) : a(p1), b(p2), c(p3) {
			edgeA = abs(b - c);
			edgeB = abs(c - a);
			edgeC = abs(a - b);
			angleA = getAngle(edgeA, edgeB, edgeC);
			angleB = getAngle(edgeB, edgeC, edgeA);
			angleC = getAngle(edgeC, edgeA, edgeB);
		}
		
		double getCircumscribedCircleRadius() {
			return edgeA / sin(angleA) / 2;
		}
		
		P getCircumscribedCirclePoint() {
			double A = sin(2 * angleA);
			double B = sin(2 * angleB);
			double C = sin(2 * angleC);
			P p;
			p.X = (a.X * A + b.X * B + c.X * C) / (A + B + C);
			p.Y = (a.Y * A + b.Y * B + c.Y * C) / (A + B + C);
			
			return p;
		}
};

int main()
{	
	int n;
	cin >> n;
	while (n--) {
		P p1, p2, p3;
		scanf("%lf %lf %lf %lf %lf %lf", &p1.X, &p1.Y, &p2.X, &p2.Y, &p3.X, &p3.Y);
		Triangle tr(p1, p2, p3);
		P p = tr.getCircumscribedCirclePoint();
		double r = tr.getCircumscribedCircleRadius();
		
		printf("%.3f %.3f %.3f\n", p.X, p.Y, r);
	}
		
	return 0;
}

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

struct Vector2D {
	double x, y;

	Vector2D() : x(0.0), y(0.0) {}
	Vector2D(double vx, double vy) : x(vx), y(vy) {}
	Vector2D& operator+=(const Vector2D& rhs)
	{
		x += rhs.x;
		y += rhs.y;
		return *this;
	}
	Vector2D& operator-=(const Vector2D& rhs)
	{
		x -= rhs.x;
		y -= rhs.y;
		return *this;
	}
	Vector2D& operator*=(double rhs)
	{
		x *= rhs;
		y *= rhs;
		return *this;
	}
};

Vector2D operator+(const Vector2D& lhs, const Vector2D& rhs)
{
	Vector2D result(lhs);
	return result += rhs;
}

Vector2D operator-(const Vector2D& lhs, const Vector2D& rhs)
{
	Vector2D result(lhs);
	return result -= rhs;
}

Vector2D operator*(double lhs, const Vector2D& rhs)
{
	Vector2D result(rhs);
	return result *= lhs;
}

void solve(Vector2D p1, Vector2D p2, Vector2D p3, Vector2D& center, double& radius)
{
	// p3-base p1, p3-base p2
	Vector2D r1(p1 - p3), r2(p2 - p3);

	double det = r2.x * r1.y - (-r1.x) * r2.y;
	double a = (r2.x * (r2.x - r1.x) + r2.y * (r1.y - r2.y)) * 0.5 / det;

	center = p3 + (0.5 * r1 + a * Vector2D(r1.y, -r1.x));

	Vector2D r = p1 - center;
	radius = sqrt(r.x * r.x + r.y * r.y);
}

int main()
{
	int count = 0;
	std::cin >> count;
	for (int i = 0; i < count; ++i) {
		Vector2D p1,p2,p3;
		std::cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y;
		if (!std::cin.good()) {
			break;
		}
		Vector2D center;
		double radius = 0.0;
		solve(p1, p2, p3, center, radius);
		std::printf("%.3f %.3f %.3f\n", center.x, center.y, radius);
	}

	return 0;
}

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

int main(){
  double x1,x2,x3,y1,y2,y3,x,n;

  cin >> n;
  int i=0;
  while(i < n){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double sina,R,BC = sqrt((x2-x3)*(x2-x3)+(y2-y3)*(y2-y3)),p,q;
    sina = ((x2-x1)*(x3-x1) + (y2-y1)*(y3-y1)) / sqrt(((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1))*((x3-x1)*(x3-x1)+(y3-y1)*(y3-y1)));

    p = ((y1-y3)*(y1*y1 - y2*y2 + x1*x1 - x2*x2) - (y1-y2)*(y1*y1-y3*y3+x1*x1 -x3*x3))/(2*(y1-y3)*(x1-x2)-2*(y1-y2)*(x1-x3));
    q = ((x1-x3)*(x1*x1 - x2*x2 + y1*y1 - y2*y2) - (x1-x2)*(x1*x1-x3*x3+y1*y1 -y3*y3))/(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));
    R = BC/(2*sina);
    printf("%.3lf %.3lf %.3lf\n",p,q,R);
    i++;
  }
}