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 <iomanip>
#include <cmath>
using namespace std;

double get_angle( double x1, double y1, double x2, double y2 )
{
	double angle;
	double m;
	if ( x1 == x2 ) angle = 90.0;
	else
	{
		m = ( y2 - y1 ) / ( x2 - x1 );
		angle = atan( m ) * 180.0 / M_PI;
	}
	return angle;
}

double get_radian( double x1, double y1, double x2, double y2 )
{
	if ( x1 == x2 ) return M_PI / 180.0;
	return atan( ( y2 - y1 ) / ( x2 - x1 ) );
}

double get_distance( double x1, double y1, double x2, double y2 )
{
	return sqrt( ( x2 - x1 ) * ( x2 - x1 ) + ( y2 - y1 ) * ( y2 - y1 ) );
}

int main( void )
{
	int n; cin >> n;
	
	double x1, y1, x2, y2, x3, y3;
	for ( int i = 0; i < n; i++ )
	{
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		double x12 = ( x1 + x2 ) / 2.0;
		double y12 = ( y1 + y2 ) / 2.0;
		double a12 = get_angle( x1, y1, x2, y2 ) - 90.0;
		double x13 = ( x1 + x3 ) / 2.0;
		double y13 = ( y1 + y3 ) / 2.0;
		double a13 = get_angle( x1, y1, x3, y3 ) - 90.0;

		double t12 = tan( a12 * M_PI / 180.0 );
		double t13 = tan( a13 * M_PI / 180.0 );
		double x = ( t12 * x12 - t13 * x13 + y13 - y12 ) / ( t12 - t13 );
		double y = t13 * ( x - x13 ) + y13;
		double r = get_distance( x1, y1, x, y );

		cout.setf( ios::fixed );
		cout.precision( 3 );
		cout << x << " " << y << " " << r << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <complex>
#include <cmath>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef complex<double> P;

double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}
P outcircle(P a,P b,P c){
  return (norm(b-c)*(norm(c-a)+norm(a-b)-norm(b-c))*a+
	  norm(c-a)*(norm(a-b)+norm(b-c)-norm(c-a))*b+
	  norm(a-b)*(norm(b-c)+norm(c-a)-norm(a-b))*c)/(P)(4*cross(b-a,c-a)*cross(b-a,c-a));
}

int main(){
  int n;
  double x1,y1,x2,y2,x3,y3;
  P a,b,c;
  cin>>n;
  for(;n>0;n--){
    cin >>x1 >>y1 >>x2 >>y2 >>x3 >>y3;
    P a=P(x1,y1);
    P b=P(x2,y2);
    P c=P(x3,y3);
    P q = outcircle(a,b,c);
    printf("%.3f %.3f %.3f\n",q.real(),q.imag(),abs(a-q));
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int main(void){
    int n;
    double x1, y1, x2, y2, x3, y3, xm1, ym1, xm2, ym2, xc, yc;
    cin >> n;
    while(n--){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        xm1=(x1+x2)/2.0;
        ym1=(y1+y2)/2.0;
        xm2=(x1+x3)/2.0;
        ym2=(y1+y3)/2.0;
        //cout << xm1 << " "<< ym1 << " " << xm2 << " " << ym2 <<"\n";
        if(y2==y1 && y3!=y1){
            double a2=-(x3-x1)/(y3-y1);
            xc=xm1;
            yc=a2*(xc-xm2)+ym2;
            double r=sqrt((x1-xc)*(x1-xc)+(y1-yc)*(y1-yc));
            printf("%.3f %.3f %.3f\n", xc, yc, r);
        }
        else if(y2!=y1 && y3==y1){
            double a1=-(x2-x1)/(y2-y1);
            xc=xm2;
            yc=a1*(xc-xm1)+ym1;
            double r=sqrt((x1-xc)*(x1-xc)+(y1-yc)*(y1-yc));
            printf("%.3f %.3f %.3f\n", xc, yc, r);
        }
        else{
            double a1=-(x2-x1)/(y2-y1);
            double a2=-(x3-x1)/(y3-y1);
            xc=(a1*xm1-a2*xm2-ym1+ym2)/(a1-a2);
            yc=a1*(xc-xm1)+ym1;
            double r=sqrt((x1-xc)*(x1-xc)+(y1-yc)*(y1-yc));
            printf("%.3f %.3f %.3f\n", xc, yc, r);
        }
    }
    
}

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

using namespace std;

int n;
double x[4],y[4],r;

int main(){
	cin >> n;
	while(n--){
		for(int i=0;i<3;i++){
			cin >> x[i] >> y[i];
		}
		if(2*(y[2]-y[0])*(x[1]-x[0])==(y[1]-y[0])*(x[2]-x[0])){
			double z=x[0];
			x[0]=x[2];
			x[2]=z;
			z=y[0];
			y[0]=y[2];
			y[2]=z;
		}
		x[3]=((y[2]-y[1])*(x[1]-x[0])*(x[2]-x[0])-(y[1]-y[0])*(x[1]+x[0])*(x[2]-x[0])+(y[2]-y[0])*(x[1]-x[0])*(x[2]+x[0]))/(2*(y[2]-y[0])*(x[1]-x[0])-(y[1]-y[0])*(x[2]-x[0]));
		if(x[0]==x[1]){
			double z=x[0];
			x[0]=x[2];
			x[2]=z;
			z=y[0];
			y[0]=y[2];
			y[2]=z;
		}
		y[3]=(y[0]+y[1])/2-(y[1]-y[0])*(x[3]-(x[0]+x[1])/2)/(x[1]-x[0]);
		r=sqrt(pow((x[3]-x[2]),2)+pow((y[3]-y[2]),2));
		printf("%.3lf %.3lf %.3lf\n",x[3]+0.00001,y[3]+0.00001,r);
	}
	return 0;
}