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 <cstdio>
#include <cstring>
#include <algorithm>
#include <complex>
using namespace std;

#define x real()
#define y imag()
typedef complex<double> Point;

Point circumscribed_Circle_Center(Point a, Point b, Point c){
  double A = a.x - b.x;
  double B = a.y - b.y;
  double C = -1 * (a.x*a.x - c.x*c.x + a.y*a.y - b.y*b.y);
  double D = a.x - c.x;
  double E = a.y - c.y;
  double F = -1 * (a.x*a.x - c.x*c.x + a.y*a.y - c.y*c.y);

  double L = (C*E - B*F) / (E*A - B*D);
  double M = (D*C - A*F) / (D*B - E*A);
  double xp = L / -2.0;
  double yp = M / -2.0;
  return (Point(xp, yp));
}

double circumscribed_Circle_Radius(Point a, Point c){
  double L = c.x * -2.0;
  double M = c.y * -2.0;
  double N = -1 * (a.x*a.x + a.y*a.y + L*a.x + M*a.y);
  double r = sqrt(L*L + M*M - 4*N) / 2.0;
  return r;
}

int main(){
  int n;
  Point a, b, c;
  cin >> n;
  while(n--){
    cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;
    Point C = circumscribed_Circle_Center(a, b, c);
    double r = circumscribed_Circle_Radius(a, C);
    printf("%0.3f %0.3f %0.3f\n", C.x, C.y, r);
  }
  return 0;
}

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

int main()
{
	int n;
	double x1,x2,x3,y1,y2,y3,x,y,r;
	double a,b,c;
	scanf("%d",&n);
	for(int i=0;i<n;i++){
		scanf("%lf%lf%lf%lf%lf%lf",&x1,&x2,&x3,&y1,&y2,&y3);
	    x=(y1-y3)*(y1*y1-y2*y2+x1*x1-x2*x2)-(y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3);
	    x/=(2*(y1-y3)*(x1-x2)-2*(y1-y2)*(x1-x3));

	    y=(x1-x3)*(x1*x1-x2*x2+y1*y1-y2*y2)-(x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3);
	    y/=(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));

	    a=(x2-x1)*(x2-x1)+(y2-y1)*(y2-y1);
	    b=(x3-x2)*(x3-x2)+(y3-y2)*(y3-y2);
	    c=(x1-x3)*(x1-x3)+(y1-y3)*(y1-y3);
	    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));

		printf("%.3f %.3f %.3f\n",x,y,r);
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>
using namespace std;
long double ff(long double a){
  
  a*=10000.0;
  //  cout<<a<<endl;
  int c = a;
  if(c%10>4)c+=10;
  c/=10;
  return (long double)c/1000.0;
}



long double ax,ay,bx,by,cx,cy,ox,oy,dx,dy,ex,ey;
//ack はACの傾き  eos はEOの切片
long double abk,ack,dok,eok,eos,dos,eps=0.0;
int n;
int main(){
  // cout<<ff(999.9999)<<endl;
  cin>>n;
  for(int i=0;i<n;i++){
    cin>>ax>>ay>>bx>>by>>cx>>cy;
    //ABの中点をD,  ACの中点をE 外接円の中心をO とする
    dx=(ax+bx)/2.0;
    dy=(ay+by)/2.0;
    ex=(ax+cx)/2.0;
    ey=(ay+cy)/2.0;
    
    if(ax==bx){
      abk=(ay-by)/0.000000000001;
    }else{
      abk=(ay-by)/(ax-bx);
    }
    if(abk==0.0)abk=0.000000000001;
    dok=-1.0/abk;
    dos=dy-dx*dok;
    
    
    if(ax==cx){
      ack=(ay-cy)/0.000000000001;
    }else{
      ack=(ay-cy)/(ax-cx);
    }
    if(ack==0.0)ack=0.000000000001;
    eok=-1.0/ack;
    eos=ey-ex*eok;
    
    //Y  =  dok X  + dos
    //Y  =  eok X  + eos
    //dok X + dos = eok X + eos
    
    long double Y,X;
    if(dok-eok==0.0){
      X=(eos-dos)/0.000000000001;
    }else{
      X=(eos-dos)/(dok-eok);
    }
    Y= dok * X + dos;
    
    printf("%.3Lf %.3Lf ",X,Y);
    //cout<<ax<<' '<<X<<' '<<ay<<' '<<Y<<endl;
    printf("%.3Lf\n" ,   sqrt( pow(ax-X,2) + pow(ay-Y,2) ));

    
    
  }
  return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>
#include <vector>
using namespace std;

#define N 10000
#define INF 200000000

int main()
{
    int n;
    vector<double> x(3), y(3), x2(3), y2(3);
    cin >> n;

    for(int p=0; p<n; p++)
    {
        cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
        for(int i=0; i<3; i++) { x2[i] = x[i]*x[i]; y2[i] = y[i]*y[i]; }

        double a, b, c, a2, b2, c2;
        a2 = abs((x[2]-x[1])*(x[2]-x[1]) + (y[2]-y[1])*(y[2]-y[1]));
        b2 = abs((x[0]-x[2])*(x[0]-x[2]) + (y[0]-y[2])*(y[0]-y[2]));
        c2 = abs((x[1]-x[0])*(x[1]-x[0]) + (y[1]-y[0])*(y[1]-y[0]));
        a = sqrt(a2); b = sqrt(b2); c = sqrt(c2);

        double s, S2;
        s = (a + b + c) / 2;
        S2 = s * (s-a) * (s-b) * (s-c);

        double cx, cy;
        cx = (a2*(b2+c2-a2)*x[0] + b2*(c2+a2-b2)*x[1] + c2*(a2+b2-c2)*x[2]) / (16 * S2);
        cy = (a2*(b2+c2-a2)*y[0] + b2*(c2+a2-b2)*y[1] + c2*(a2+b2-c2)*y[2]) / (16 * S2);

        cout << setprecision(3) << fixed <<
            cx << " " << cy << " " << sqrt(abs((cx-x[0])*(cx-x[0])+(cy-y[0])*(cy-y[0]))) << endl;
    }
}