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>
#include <math.h>

int main(void) {
  int i, n, j, k;
  double x1, y1, x2, y2, x3, y3;
  scanf("%d", &n);
  for(i = 0; i < n; ++i) {
    scanf("%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3);
    double xt1 = (x1 + x2) / 2, xt2 = (x3 + x2) / 2, yt1 = (y1 + y2) / 2, yt2 = (y3 + y2) / 2;
    double t1 = y1 - y2, t2 = y2 - y3;
    double x, y, k1, k2;
    if(!t1) {
      x = xt1;
      k2 = -(x2 - x3) / (y2 - y3);
      y = k2 * (x - xt2) + yt2;
    } else if(!t2) {
      x = xt2;
      k1 = -(x1 - x2) / (y1 - y2);
      y = k1 * (x - xt1) + yt1;
    } else {
      k1 = -(x1 - x2) / (y1 - y2), k2 = -(x2 - x3) / (y2 - y3);
      x = ((k1 * xt1 - k2 * xt2) - (yt1 - yt2)) / (k1 - k2);
      y = k1 * (x - xt1) + yt1;
      //printf("%f %f ", k1, k2);
    }
    double r = sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1));
    //printf("%f %f %f %f\n", xt1, yt1, xt2, yt2);
    printf("%.3f %.3f %.3f\n", x, y, r);
  }
  return 0;
}

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

using namespace std;

int main()
{
	int n; cin >>n;
	int i;
	for(i=0;i<n;i++){
		double x1,y1,x2,y2,x3,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 x = (b1*c2-b2*c1)/(a1*b2-a2*b1);
		double y = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		double r = sqrt( (x-x1)*(x-x1)+(y-y1)*(y-y1) );
		
		// 小数点第４位を四捨五入
		x = floor(1000*x + 0.5)/1000;
		y = floor(1000*y + 0.5)/1000;
		r = floor(1000*r + 0.5)/1000;
		
		printf("%.3lf %.3lf %.3lf\n", x,y,r);
	}
	return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define EPS (1e-10)
#define equals(a, b) (fabs(a - b) < EPS)

struct Point {
    double x, y;

    Point(){}
    Point(double x, double y) : x(x), y(y) {}

    Point operator + (const Point &p) const {
        return Point(x + p.x, y + p.y);
    }

    Point operator - (const Point &p) const {
        return Point(x - p.x, y - p.y);
    }
    
    Point operator * (const double &k) const {
        return Point(x * k, y * k);
    }
};

double dot(const Point &a, const Point &b)
{
    return a.x * b.x + a.y * b.y;
}

double norm(const Point &p)
{
    return dot(p, p);
} 

double abs(const Point &p)
{
    return sqrt(norm(p));
}

typedef vector<Point> Polygon;

struct Circle {
    Point p;
    double r;
    Circle(Point p, double r) : p(p), r(r) {}
};

bool check(const Polygon &pg, const Point &p)
{
    double r = abs(pg[0] - p);
    for (int i = 1; i < 3; i++) {
        if (!equals(r, abs(pg[i] - p))) return 0;
    }
    return 1;
}

Circle smallest_enclosing_circle(const Polygon &pg)
{    
    Point p(0, 0);
    int N = 3;
    for (int i = 0; i < (int)pg.size(); i++) {
	p = p + pg[i];
    }
    p = p * (1. / pg.size());
    double d = 0.5;
    while (!check(pg, p)) {
	for (int i = 0; i < 150; i++) {
	    int s = 0;
	    for (int j = 0; j < N; j++) {
		if (abs(pg[s] - p) - abs(pg[j] - p) < -EPS) {
		    s = j;
		}
	    }
	    p = p + (pg[s] - p) * d;
	}
	d *= 0.5;
    }
    double r = abs(p - pg[0]);
    return Circle(p, r);
}

int main()
{
    Polygon pg(3);
    int N;
    cin >> N;
    while (N--) {
        for (int i = 0; i < 3; i++) {
            cin >> pg[i].x >> pg[i].y;
        }
        auto c = smallest_enclosing_circle(pg);
        printf("%.3f %.3f %.3f\n", c.p.x, c.p.y, c.r);
    }
    return 0;
}

C++: #include <iostream>
#include <queue>
#include <vector>
#include <math.h>
#include <algorithm>
#include <iomanip>
#include <string>
using namespace std;

int main(){
	int n;
	double xa, ya, xb, yb, xc, yc;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		cin >> xa >> ya >> xb >> yb >> xc >> yc;
		double a, b, c;
		a = (xb - xc) * (xb - xc) + (yb - yc) * (yb - yc);
		b = (xc - xa) * (xc - xa) + (yc - ya) * (yc - ya);
		c = (xa - xb) * (xa - xb) + (ya - yb) * (ya - yb);
		double px, py, r;
		px = a * (b + c - a) * xa + b * (c + a - b) * xb + c * (a + b - c) * xc;
		py = a * (b + c - a) * ya + b * (c + a - b) * yb + c * (a + b - c) * yc;
		px /= a * (b + c - a) + b * (c + a - b) + c * (a + b - c);
		py /= a * (b + c - a) + b * (c + a - b) + c * (a + b - c);
		a = sqrt(a);
		b = sqrt(b);
		c = sqrt(c);
		r = a * b * c / sqrt((a + b + c) * (-a + b + c) * (a - b + c) * (a + b - c));
		cout << fixed << setprecision(3) << px << ' ' << py << ' ' << r << endl;
	}
	return 0;
}