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>
using namespace std;

class Point {
public:
  double x, y;
  Point(double x=0, double y=0) : x(x), y(y) {}
};

double dist(Point p1, Point p2) {
  return sqrt(pow(p1.x-p2.x, 2.0)+pow(p1.y-p2.y, 2.0));
}

Point circumcenter(Point pa, Point pb, Point pc) {
  double A = abs(atan2(pb.y-pa.y, pb.x-pa.x) - atan2(pc.y-pa.y, pc.x-pa.x));
  double B = abs(atan2(pa.y-pb.y, pa.x-pb.x) - atan2(pc.y-pb.y, pc.x-pb.x));
  double C = abs(atan2(pa.y-pc.y, pa.x-pc.x) - atan2(pb.y-pc.y, pb.x-pc.x));
  double a = dist(pb, pc);
  double b = dist(pa, pc);
  double c = dist(pa, pb);
  return Point((pa.x*sin(A*2)+pb.x*sin(B*2)+pc.x*sin(C*2))
	       /(sin(A*2)+sin(B*2)+sin(C*2)),
	       (pa.y*sin(A*2)+pb.y*sin(B*2)+pc.y*sin(C*2))
	       /(sin(A*2)+sin(B*2)+sin(C*2))
	       );
}

main() {
  Point p[3];
  Point op;
  int n;
  cin >> n;
  for(int i = 0; i < n; i++) {
    for(int j = 0; j < 3; j++) cin >> p[j].x >> p[j].y;
    op = circumcenter(p[0], p[1], p[2]);
    /*double A = abs(atan2(p[1].y-p[0].y, p[1].x-p[0].x)
		   - atan2(p[2].y-p[0].y, p[2].x-p[0].x));*/
    printf("%.3f %.3f %.3f\n", op.x, op.y, dist(op,p[0]));
  }
}

C++: #include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <map>
#include <algorithm>

#define rep(i, j) FOR(i, 0, j)
#define FOR(i, j, k) for(int i = j; i < k; ++i)
#define SQ(i) ((i) * (i))
using namespace std;

int main(){
  double x1, y1, x2, y2, x3, y3;
  double a1, b1, c1, a2, b2, c2, x, y, a, b, c, cos_a, sin_a, r;
  int n;
  scanf("%d", &n);
  while(n--){
    scanf("%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3);
    a1 = 2 * (x2 - x1); a2 = 2 * (x3 - x1);
    b1 = 2 * (y2 - y1); b2 = 2 * (y3 - y1);
    c1 = SQ(x1) - SQ(x2) + SQ(y1) - SQ(y2);
    c2 = SQ(x1) - SQ(x3) + SQ(y1) - SQ(y3);
    x = (b1 * c1 - b2 * c1) / (a1 * b2 - a2 * b1);
    y = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);

    r = SQ(x3-x) + SQ(y3 - y);
    r = sqrt(r);
    printf("%.3f %.3f %.3f\n",x, y, r);
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int INF = 1000000000;
#define REP(i,s,n) for(int i=(int)(s);i<(int)(n);i++)
#define rep(i,n) REP(i, 0, n)
typedef complex<double> Point;
const double EPS = 1.0e-9;
const double PI = acos(-1);
namespace std {
    bool operator < (const Point& a, const Point& b){
        return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
    }
}
struct Line : public vector<Point> {
    Line(const Point& a, const Point& b){
        push_back(a);
        push_back(b);
    }
};
double cross(const Point& a, const Point& b){
    return imag(conj(a) * b);
}
Point cross_point(const Line& l, const 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]; // same line
    if (abs(A) < EPS) assert(false); // !!!PRECONDITION NOT SATISFIED!!!
    return m[0] + B / A * (m[1] - m[0]);
}
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int test; cin >> test;
    rep(casenum, test){
        Point a, b, c, m1, m2, d1, d2;
        cin >> a.real() >> a.imag() >> b.real() >> b.imag() >> c.real() >> c.imag();
        m1 = (a + b) / 2.0, m2 = (b + c) / 2.0;
        d1 = polar(1.0, arg(b - a) + PI / 2.0), d2 = polar(1.0, arg(c - b) + PI / 2.0);
        Line l1(m1, m1 + d1), l2(m2, m2 + d2);
        Point ans = cross_point(l1, l2);
        double r = abs(ans - a);
        printf("%.3lf %.3lf %.3lf\n", ans.real(), ans.imag(), r);
    }
    return 0;
}