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;
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;
	}
	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 <iostream>
#include <cmath>

using namespace std;

int n;
double tx1, ty1, tx2, ty2, tx3, ty3;

int main()
{
	cin >> n;
	for(int i=0; i<n; i++)
	{
		cin >> tx1 >> ty1;
		cin >> tx2 >> ty2;
		cin >> tx3 >> ty3;

		double mx1 = (tx1 + tx2) / 2.0;
		double my1 = (ty1 + ty2) / 2.0;
		double a1 = (double)(tx2 - tx1);
		double b1 = (double)(ty2 - ty1);
		double c1 = a1 * mx1 + b1 * my1;

		double mx2 = (tx2 + tx3) / 2.0;
		double my2 = (ty2 + ty3) / 2.0;
		double a2 = (double)(tx3 - tx2);
		double b2 = (double)(ty3 - ty2);
		double c2 = a2 * mx2 + b2 * my2;

		if(c1 == 0 && c2 == 0)
		{
			printf("%.3f %.3f %.3f\n", (double)tx1, (double)ty1, 0.0);
			continue;
		}

		double det = a1 * b2 - a2 * b1;
		if(det == 0)
		{
			printf("can not solve\n");
			continue;
		}
		
		double cx = (b2 * c1 - a2 * c2) / det;
		double cy = (-a2 * c1 + a1 * c2) / det;
		
		double dx = cx - tx1;
		double dy = cy - ty1;
		double r = sqrt(dx*dx + dy*dy);
		
		printf("%.3f %.3f %.3f\n", cx, cy, r);
	}
}

C++: #include <stdio.h>
#include <math.h>

float round(float val, int d) {
	return (int)(val * pow(10, d) + 0.5) / pow(10,d);
}

int main () {
	int n;
	float x1, x2, x3, y1, y2, y3;
	float m1, m2, b1, b2;
	float x, y, r, hx, hy;
	scanf("%d", &n);
	while (n--) {
		scanf("%f %f %f %f %f %f" , &x1, &y1, &x2, &y2, &x3, &y3);
		if (y1 == y2) {
			x = (x1 + x2) / 2.0;
			m2 = (y3 == y1) ? 0 : 1 / ((y3 - y1) / (x3 - x1));
			hx = (x1 + x3) / 2.0;
			hy = (y1 + y3) / 2.0;
			b2 = m2 * hx + hy;
			y = -m2 * x + b2;
		} else if (y1 == y3) {
			x = (x1 + x3) / 2.0;
			m1 = (y2 == y1) ? 0 : 1 / ((y2 - y1) / (x2 - x1));
			hx = (x1 + x2) / 2.0;
			hy = (y1 + y2) / 2.0;
			b1 = m1 * hx + hy;
			y = -m1 * x + b1;
		} else {
			m1 = (x2 == x1) ? 0 : 1 / ((y2 - y1) / (x2 - x1));
			m2 = (x3 == x1) ? 0 : 1 / ((y3 - y1) / (x3 - x1));
			hx = (x1 + x2) / 2.0;
			hy = (y1 + y2) / 2.0;
			b1 = m1 * hx + hy;
			hx = (x1 + x3) / 2.0;
			hy = (y1 + y3) / 2.0;
			b2 = m2 * hx + hy;
			x = (b1 - b2) / (m1 - m2);
			y = -m1 * x + b1;
		}
		x = round(x, 3);
		y = round(y, 3);
		r = round(sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1)), 3);
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}

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

using namespace std;

int main(){
	int n;

	cin >> n;

	double* x1 = new double[ n ];
	double* y1 = new double[ n ];
	double* x2 = new double[ n ];
	double* y2 = new double[ n ];
	double* x3 = new double[ n ];
	double* y3 = new double[ n ];

	for ( int i = 0; i < n; ++i ){
		cin >> x1[ i ] >> y1[ i ]
			>> x2[ i ] >> y2[ i ]
			>> x3[ i ] >> y3[ i ];
	}

	for ( int i = 0; i < n; ++i ){
		double x12, y12, b12, x23, y23, b23, ox, oy, r;
		x12 = x2[ i ] - x1[ i ];
		y12 = y2[ i ] - y1[ i ];
		b12 = x12 * ( x1[ i ] + x2[ i ] ) / 2.0 + y12 * ( y1[ i ] + y2[ i ] ) / 2.0;
		x23 = x3[ i ] - x2[ i ];
		y23 = y3[ i ] - y2[ i ];
		b23 = x23 * ( x2[ i ] + x3[ i ] ) / 2.0 + y23 * ( y2[ i ] + y3[ i ] ) / 2.0;
		oy = ( x12 * b23 - x23 * b12 ) / ( x12 * y23 - x23 * y12 );
		if ( x12 ){
			ox = ( b12 - y12 * oy ) / x12;
		}else{
			ox = ( b23 - y23 * oy ) / x23;
		}
		r = sqrt( pow( ox - x1[ i ], 2.0 ) + pow( oy - y1[ i ], 2.0 ) );

		cout << showpoint << setprecision( 3 ) << setiosflags( ios::fixed )
			 << ( ox == -0.0 ? 0.0 : ox ) << " "
			 << ( oy == -0.0 ? 0.0 : oy ) << " "
			 << r << endl;
	}

	delete[] x1, y1, x2, y2, x3, y3;
	return 0;
}