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 <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <cstring>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>

using namespace std;

static const double EPS = 1e-9;
typedef long long ll;
typedef pair<int, int> paii;
typedef complex<double> P;

double ccw(P& p1, P& p2, P& p3)
{
	return ((p2.real() - p1.real())*(p3.imag() - p1.imag()) 
				- (p2.imag() - p1.imag())*(p3.real() - p1.real())) > 0;
}// ccw>0 ...ccw, ccw=0 ...colinear, ccw<0 ...cw

int main()
{
	int n;
	cin >> n;
	
	while(n--)
	{
		double x1, x2, x3, y1, y2, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		
		double a1 = 2*(x2-x1);
		double b1 = 2*(y2-y1);
		double c1 = x1*x1-x2*x2+y1*y1-y2*y2;
		double a2 = 2*(x3-x1);
		double b2 = 2*(y3-y1);
		double c2 = x1*x1-x3*x3+y1*y1-y3*y3;
		
		double xp = (b1*c2-b2*c1)/(a1*b2-a2*b1);
		double yp = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		double r = hypot((xp-x1), (yp-y1));
		
		cout.precision(3);
		printf("%.3f %.3f %.3f\n", xp,yp, r);
	}
	
	return 0;
}

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

typedef complex<double> P;
#define PI 3.14159265358979
#define EPS (1e-10)
#define X real()
#define Y imag()

class Triangle{
  private:
    P a, b, c;
    double edgeA, edgeB, edgeC;
    double angleA, angleB, angleC;
    
    double LawOfCosines(double a, double b, double c){
      return acos((b*b + c*c - a*a) / (2.0*b*c));
    }
    
    bool equal(double a, double b){
      return (abs(a - b) < EPS) ? true : false;
    }
  
  public:
    Triangle(P p1, P p2, P p3){
      a = p1; b = p2; c = p3;
      edgeB = abs(c - a);
      edgeC = abs(a - b);
      edgeA = abs(b - c);
      angleA = LawOfCosines(edgeA, edgeB, edgeC);
      angleB = LawOfCosines(edgeB, edgeC, edgeA);
      angleC = LawOfCosines(edgeC, edgeA, edgeB);
    }
    
    double circumscribedCircleRedius(){
      return (edgeA / sin(angleA) / 2.0);
    }
    
    double circumscribedCircleX(){
      double A = sin(2.0*angleA);
      double B = sin(2.0*angleB);
      double C = sin(2.0*angleC);
      return ((a.X*A + b.X*B + c.X*C) / (A + B + C));
    }
    
    double circumscribedCircleY(){
      double A = sin(2.0*angleA);
      double B = sin(2.0*angleB);
      double C = sin(2.0*angleC);
      return ((a.Y*A + b.Y*B + c.Y*C) / (A + B + C));
    }
};

int main(void){
  int n;
  double x1, x2, x3, y1, y2, y3;
  double r, xp, yp;

  cin >> n;
  for(int i = 0; i < n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    P a(x1, y1);
    P b(x2, y2);
    P c(x3, y3);
    Triangle Tr(a, b, c);
    r = Tr.circumscribedCircleRedius();
    xp = Tr.circumscribedCircleX();
    yp = Tr.circumscribedCircleY();
    printf("%.3lf %.3lf %.3lf\n", xp, yp, r);
  }
  return 0;
}

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

using namespace std;

double henkan(double r) {
	r *= 10000.0;
	if (((((int)r % 10000)% 1000) % 100) % 10 >= 5){
		r += 10.0;
		r /= 10000.0;
	} else {
		r /= 10000.0;
	}
	return r;
}

int main() {
	int n;
	cin >> n;
	double x1, y1, x2, y2, x3, y3;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double r;
		double sina;
		double cosa;
		double a = sqrt(pow(x3-x2,2)+pow(y3-y2,2));
		double b = sqrt(pow(x2-x1,2)+pow(y2-y1,2));
		double c = sqrt(pow(x3-x1,2)+pow(y3-y1,2));
		cosa = (b*b+c*c-a*a)/(2.0*b*c);
		sina = sqrt(1-cosa*cosa);
		r = a / (sina*2.0);
		double aa, bb;
		double x4, y4;
		double xx1, yy1, xx2, yy2;
		xx1 = (x1+x2)/2.0;
		xx2 = (x1+x3)/2.0;
		yy1 = (y1+y2)/2.0;
		yy2 = (y1+y3)/2.0;
		if (y1 == y2) {
			bb = - (x3-x1)/(y3-y1);
			x4 = xx1;
			y4 = bb*x4+yy2-bb*xx2;
		} else if (y1 == y3){
			aa = - (x2-x1)/(y2-y1);
			y4 = xx2;
			x4 = (y4-yy1+aa*xx1)/aa;
		} else {
			aa = - (x2-x1)/(y2-y1);
			bb = - (x3-x1)/(y3-y1);
			x4 = (aa*xx1-bb*xx2-yy1+yy2)/(aa-bb);
			y4 = aa*x4+yy1-aa*xx1;
		}
		x4 = henkan(x4);
		y4 = henkan(y4);
		r = henkan(r);
		printf("%.3f %.3f %.3f\n", x4, y4, r);
	}
	return 0;
}