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<cmath>
#include<iomanip>
using namespace std;
int main(){
  int n;
  double x[3],y[3];
  cin >> n;
  for(int i=0;i<n;i++){
    for(int i=0;i<3;i++) cin >> x[i] >> y[i];
    double a1 = 2 * (x[1] - x[0]);
    double b1 = 2 * (y[1] - y[0]);
    double c1 = x[0] * x[0] - x[1] * x[1] + y[0] * y[0] - y[1] * y[1];
    double a2 = 2 * (x[2] - x[0]);
    double b2 = 2 * (y[2] - y[0]);
    double c2 =  x[0] * x[0] - x[2] * x[2] + y[0] * y[0] - y[2] * y[2];
    double ansx = (b1*c2-b2*c1)/(a1*b2-a2*b1);
    double ansy = (c1*a2-c2*a1)/(a1*b2-a2*b1);
    double ans = sqrt((ansx-x[0])*(ansx-x[0])+(ansy-y[0])*(ansy-y[0]));
    cout << fixed << setprecision(3) << ansx << ' ' << ansy << ' ' << ans << endl;
  }
  return 0;
}

C++: #include <cstdio>
#include <cassert>
#include <cmath>
#include <complex>
#include <vector>
#include <utility>
using namespace std;

typedef complex<double> Point;
#define X real()
#define Y imag()
#define EPS 10e-8

namespace std {
  bool operator < (Point a, Point b) {
    if (a.X != b.X) return a.X < b.X;
    return a.Y < b.Y;
  }
}

double cross(Point a, Point b) {
  return (conj(a)*b).Y;
}

double dot(Point a, Point b) {
  return (conj(a)*b).X;
}

double dist2(Point a, Point b) {
  return dot(a-b, a-b);
}

struct Line : public vector<Point> {
  Line(Point a, Point b) {push_back(a); push_back(b);}
};

int n;
Point ps[3];

Line PerpendBisect(Line l) {
  Point mid = (l[0]+l[1])/2.0;
  return Line(mid, mid+(l[1]-l[0])*Point(0, 1));
}

Point GetCrossPoint(Line l, Line m) {
  double A = cross(l[1]-l[0], m[1]-m[0]);
  double B = cross(l[1]-l[0], l[1]-m[0]);
  if (abs(A) < EPS && abs(B) < EPS) return m[0];
  if (abs(A) < EPS) assert(false);
  return m[0] + B / A * (m[1]-m[0]);
}

double round3(double t) {
  return int(t*1000+0.5)/1000.0;
}

int main() {
  scanf("%d", &n);
  while (n--) {
    for (int i=0; i<3; i++) {
      double x, y;
      scanf("%lf%lf", &x, &y);
      ps[i] = Point(x, y);
    }

    Line l(ps[0], ps[1]);
    Line m(ps[1], ps[2]);
    Point p = GetCrossPoint(PerpendBisect(l), PerpendBisect(m));
    double r = sqrt(dist2(p, ps[0]));
    printf("%.3f %.3f %.3f\n", round3(p.X), round3(p.Y), round3(r));
  }
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <list>
#include <algorithm>
#include <cmath>
#include <stack>
#include <set>
using namespace std;

#define REP(i,n) for(int (i)=0; (i)<(n); (i)++)
#define FOR(i,a,b) for(int (i)=(a); (i)<(b); (i)++)
#define PUSH(n,v) for(int i=0; i<(n); i++) {int j; cin >> j; v.push_back(j);}
#define ALL(v) v.begin(), v.end()

const double ERR = 1e-9;

typedef pair<double, double> XY;

XY solveeq(double a, double b, double c, double d, double e, double f) {
    double x, y;
    x = (c*e - b*f)/(a*e - b*d);
    y = (a*f - c*d)/(a*e - b*d);
    return make_pair(x,y);
}

XY getcenter(double x1, double y1, double x2, double y2, double x3, double y3) {
    return solveeq((x2-x1)/(y2-y1+ERR), 1, (x2*x2-x1*x1)/2/(y2-y1+ERR)+(y1+y2)/2, (x3-x2)/(y3-y2+ERR), 1, (x3*x3-x2*x2)/2/(y3-y2+ERR)+(y2+y3)/2);
}

double getradius(XY center, double x1, double y1) {
    return sqrt((x1-center.first)*(x1-center.first) + (y1-center.second)*(y1-center.second));
}

int main() {
    double x1,y1,x2,y2,x3,y3;
    int n;
    cin >> n;
    REP(i,n) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        XY center = getcenter(x1, y1, x2, y2, x3, y3);
        double radius = getradius(center, x1, y1);
        printf("%.3f %.3f %.3f\n", center.first+ERR, center.second+ERR, radius+ERR);
    }
    return 0;
}

C++: //04
#include<iostream>
#include<complex>
#include<iomanip>
#include<cmath>
#include<algorithm>

#define S(X) ((X)*(X))

using namespace std;

int main(){
  int n;
  cin>>n;
  for(int i=0;i<n;i++){
    complex<double> p[3];
    for(int j=0;j<3;j++){
      double x,y;
      cin>>x>>y;
      p[j]=complex<double>(x,y);
    }
    complex<double> t[2];
    t[0]=p[1]-p[0];
    t[1]=p[2]-p[0];
    if(t[0].real()*t[1].imag()-t[0].imag()*t[1].real()<0){
      swap(p[1],p[2]);
    }
    double d[3];
    for(int j=0;j<3;j++){
      d[j]=abs(p[j]-p[(j+1)%3]);
    }
    double r=d[0]/2/sqrt(1-S((S(d[1])+S(d[2])-S(d[0]))/2/d[1]/d[2]));
    complex<double> c=p[0]+polar(r,arg(p[1]-p[0])+acos(d[0]/2/r));
    cout<<fixed<<setprecision(3)<<c.real()<<' '<<c.imag()<<' '<<r<<endl;
  }
  return 0;
}