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 <bits/stdc++.h>

using namespace std;

#define EPS (1e-5)
#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<iomanip>
#include<string>
#include<algorithm>
#include<functional>
#include<vector>
#include<math.h>

using namespace std;

typedef long long int lli;

int main() {
	int n;
	double xy[3][3],x,y;
	double a, b, c, d, e, f, radius;
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < 3; j++) {
			for (int k = 0; k < 2; k++) {
				cin >> xy[j][k];
			}
		}
		a = -2 * (xy[0][0]-xy[1][0]);
		b = -2 * (xy[0][1] - xy[1][1]);
		c = -1 * (pow(xy[0][0], 2) + pow(xy[0][1], 2)) + pow(xy[1][0], 2) + pow(xy[1][1], 2);
		d = -2 * (xy[0][0] - xy[2][0]);
		e = -2 * (xy[0][1] - xy[2][1]);
		f = -1 * (pow(xy[0][0], 2) + pow(xy[0][1], 2)) + pow(xy[2][0], 2) + pow(xy[2][1], 2);
		y = (a*f - c*d) / (a*e - b*d);
		x = (c*e - b*f) / (a*e - b*d);
		if (x == 0) x = 0;
		if (y == 0) y = 0;
		radius =abs(sqrt((x-xy[0][0])+(y-xy[0][1])));

		cout << fixed << setprecision(3) << x << " " << y << " "<< radius <<endl;
	}
	return 0;
}

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

using namespace std;
int main(){
  int n;cin >> n;
  double x1,x2,x3,y1,y2,y3,x12,x13,y12,y13,xx12,yy12,xx13,yy13,x,y,r;
  for(int i=0;i<n;i++){
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    x12 = x1 -x2;
    x13 = x1 -x3;
    y12 = y1-y2;
    y13 = y1 -y3;
    xx12 = x1*x1-x2*x2;
    xx13 = x1*x1-x3*x3;
    yy12 = y1*y1-y2*y2;
    yy13 = y1*y1-y3*y3;

    x=-0.5*(y12*(xx13+yy13)-y13*(xx12+yy12))/(x12*y13-x13*y12);
    y=-0.5*(x12*(xx13+yy13)-x13*(xx12+yy12))/(y12*x13-y13*x12);
    r=sqrt(x*x+y*y+x1*x1+y1*y1-2*x*x1-2*y*y1);
    printf("%.3lf %.3lf %.3lf\n",x,y,r);
   }
   return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
class circle{
    public:
        void solve(double x1,double y1,double x2,double y2,double x3,double y3){
                double left,right,top,bottom,px,py,r;
                left=min(x1,min(x2,x3));
                right=max(x1,min(x2,x3));
                top=max(x1,min(x2,x3));
                bottom=min(x1,min(x2,x3));
                px=(left+right)/2;
                py=(top+bottom)/2;
                r=sqrt(pow(x1-px,2)+pow(y1-py,2));
                printf("%.3lf %.3lf %.3lf\n",px,py,r);
        }
};
int main(){
    circle inst;
    int n;
    double x1,y1,x2,y2,x3,y3;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>x1>>y1>>x2>>y2>>x3>>y3;
        inst.solve(x1,y1,x2,y2,x3,y3);
    }
}