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 <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>

using namespace std;

int main()
{

	int n;
	double a,b,c,d,e,f,x1,x2,x3,y1,y2,y3;
	double ans_x,ans_y,ans_r;

	cin>>n;

	for(int i=0;i<n;i++){

		cin>>x1>>y1>>x2>>y2>>x3>>y3;


		if(y1==y2){
			ans_x=(x1+x2)/2.0;
			a=-(x3-x2)/(y3-y2);
			b=(x2+x3)/2.0; c=(y2+y3)/2.0;
		}else if(y2==y3){
			ans_x=(x2+x3)/2.0;
			a=-(x2-x1)/(y2-y1);
			b=(x1+x2)/2.0; c=(y1+y2)/2.0;
		}else{
			a=-(x3-x2)/(y3-y2);
			b=(x2+x3)/2.0; c=(y2+y3)/2.0;
			d=-(x2-x1)/(y2-y1);
			e=(x1+x2)/2.0; f=(y1+y2)/2.0;
			
			ans_x=(a*b-d*e-c+f);
		}

		ans_y=a*(ans_x-b)+c;
		ans_r=sqrt( (x1-ans_x)*(x1-ans_x) + (y1-ans_y)*(y1-ans_y) );

		printf("%.3f %.3f %.3f\n",ans_x,ans_y,ans_r);

	}

	return 0;

}

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

using namespace std;
typedef struct { double x; double y;} point;
 
point getCircumcenter(point a, point b, point c)
{
    double A1 = 2 * ( b.x - a.x );
    double B1 = 2 * ( b.y - a.y );
    double C1 = pow(a.x,2)-pow(b.x,2) + pow(a.y,2)-pow(b.y,2);
    double A2 = 2 * ( c.x - a.x );
    double B2 = 2 * ( c.y - a.y );
    double C2 = pow(a.x,2)-pow(c.x,2) + pow(a.y,2)-pow(c.y,2);
    double X = (B1 * C2 - B2 * C1) / (A1 * B2 - A2 * B1);
    double Y = (C1 * A2 - C2 * A1) / (A1 * B2 - A2 * B1);
	point P;
	P.x=X;
	P.y=Y;
    return P;
}

double getTwoPointDictance(point a, point b)
{
	return fabs(sqrt(pow((a.x-b.x),2)+pow((a.y-b.y),2)));
}
double AntiMinusZero(double n)
{
	if(n=0)
	{
		return 0.0f;
	}
	return n;
}

int main()
{
    int n;
    cin>>n;
    point a,b,c;
    while(n--)
	{
        cin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
        point ans = getCircumcenter(a,b,c);
        printf("%.3lf %.3lf %.3lf", AntiMinusZero(ans.x), AntiMinusZero(ans.y), getTwoPointDictance( a, ans));
    }
}

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

using namespace std;

int main() {
    int n;

    scanf("%d", &n);

    double x1, y1, x2, y2, x3, y3;

    while (n--) {
        scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
 
        if (y1 == y3) {
            swap(x3, x2);
            swap(y3, y2);
        }

        if (y2 == y3) {
            swap(x3, x1);
            swap(y3, y1);
        }

        double a = x2 - x3;
        double b = y2 - y3;
        double c = x2 + x3;
        double d = y2 + y3;
        double e = x3 - x1;
        double f = y3 - y1;
        double g = x3 + x1;
        double h = y3 + y1;
        double ab = a / b;
        double d2 = d / 2.0;
        double c2 = c / 2.0;
        double ef = e / f;

        double px = (ab * c2 + d2 - ef * (g / 2) - h / 2) / (ab - ef);
        double py = -ab * px + ab * c2 + d2;
        double x = x1 - px;
        double y = y1 - py;
        double r = sqrt(x * x + y * y);

        printf("%.3lf %.3lf %.3lf\n", px, py, r);
    }

    return 0;
}

C++: // 入力された3点を通る円の中心座標と半径を求める。

#include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;

int main(void) {
    double x1, y1, x2, y2, x3, y3;
    double ox, oy, r;
    int n;
    
    cin >> n;
    for(int i = 0; i < n; ++i) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        
        ox = ((y2 - y3)*(x1*x1 - x2*x2 + y1*y1 - y2*y2) - (y1 - y2)*(x2*x2 - x3*x3 + y2*y2 - y3*y3))
             / (2*((x1 - x2)*(y2 - y3) - (x2 - x3)*(y1 - y2)));
        if(y1 != y2) {
            oy = ((x1*x1 - x2*x2 + y1*y1 - y2*y2) - 2*(x1 - x2)*ox) / (2 * (y1 - y2));
        }else {
            oy = ((x2*x2 - x3*x3 + y2*y2 - y3*y3) - 2*(x2 - x3)*ox) / (2 * (y2 - y3));
        }
        
        r = sqrt((ox - x1)*(ox - x1) + (oy - y1)*(oy - y1));
        
        printf("%.3lf %.3lf %.3lf", ox, oy, r);
        cout << endl;
    }
}

C++: #include <iostream>
#include <math.h>
using namespace std;

int main(){

	double x1,y1,x2,y2,x3,y3,x,y;
	double a1,a2,b1,b2,c1,c2;
	int n,i;
	cin >> n;
	for(i=0;i<n;i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a1 = 2*(x2-x1);
		b1 = 2*(y2-y1);
		c1 = x1*x1-x2*x2+y1*y1-y2*y2;
		a2 = 2*(x3-x1);
		b2 = 2*(y3-y1);
		c2 = x1*x1-x3*x3+y1*y1-y3*y3;
		x=(b1*c2-b2*c1)/(a1*b2-a2*b1);
		y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
		printf("%.3f %.3f %.3f\n",x,y,sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1)));
	}

	return 0;
}