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;
typedef complex<double> point;
typedef pair<point, point> line;
double cross(point a, point b){
  return imag(conj(a) * b);
}


point cross_point(line a, line b){
  double d1 = cross(b.second - b.first,
                    b.first - a.first);
  double d2 = cross(b.second - b.first,
                    a.second - a.first);
  return a.first + (a.second - a.first) * (d1/d2);
}

int main(int argc, char *argv[]){
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    point x[3];
    for (int j = 0; j < 3; j++) {
      double real, imag;
      cin >> real >> imag;
      x[j] = point(real, imag);
    }
    point m1 = (x[0] - x[1]), m2 = (x[1] - x[2]);
    m1 = point(m1.real()/2, m1.imag()/2);
    m2 = point(m2.real()/2, m2.imag()/2);
    line n1 = line(m1*point(0, 1), m1*point(0, -1)),
      n2 = line(m2*point(0, 1), m2*point(0, -1));
    n1.first += m1 + x[0];
    n1.second += -m1 + x[0];
    n2.first += m2 + x[1];
    n2.second += -m2 + x[1];
      
    point center = cross_point(n1, n2);
    center -= 
      printf("%5lf %5lf %5lf\n", center.real(), center.imag(),
             abs(center - x[0]));
  }
  return 0;
}

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

int main()
{
	int i,n;
	double x1,x2,x3,y1,y2,y3;
	double m1,m2,xm1,xm2,ym1,ym2,xo,yo,r;
	//m1,m2…辺の法線 xm1,ym1…辺の中点の座標

	cin>>n;
	for(i=0;i<n;i++){
		flg=0;
		cin>>x1>>y1>>x2>>y2>>x3>>y3;

		xm1=(x1+x2)/2;
		xm2=(x2+x3)/2;
		ym1=(y1+y2)/2;
		ym2=(y2+y3)/2;
		
		if(y1-y2!=0) 		m1=(-1)*(x1-x2)/(y1-y2);
		else if(y1-y2==0)	flg=1;
		
		if(y2-y3!=0)		m2=(-1)*(x2-x3)/(y2-y3);
		else if(y2-y3==0)	flg=2;
		
		if(flg==0) 	xo=(m1*xm1-m2*xm2+ym2-ym1)/(m1-m2);
		else if(flg==1)xo=xm1;
		else if(flg==2)xo=xm2;

		if(flg==1)yo=m2*(xo-xm2)+ym2;
		else		yo=m1*(xo-xm1)+ym1;
		r=sqrt((x1-xo)*(x1-xo)+(y1-yo)*(y1-yo));
		printf("%.3lf %.3lf %.3lf \n",xo,yo,r);
	}
}

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

int main(void) {

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

  double x12,y12,x13,y13;
  double ang12,ang13;
  double xp,yp,r;

  int flag;
  int i;

  scanf("%d",&data);

  for(i=0;i<data;i++) {
    flag=1;
    scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);

    x12=(x1+x2)/2.0;
    y12=(y1+y2)/2.0;
    x13=(x1+x3)/2.0;
    y13=(y1+y3)/2.0;

    if(y1==y2) {

      xp=x12;

      ang13=-(x3-x1)/(y3-y1);

      yp=ang13*(xp-x13)+y13;

      flag=0;

    }

    if(y1==y3) {

      xp=x13;
      
      ang12=-(x2-x1)/(y2-y1);

      yp=ang12*(xp-x12)+y12;

      flag=0;

    }

    if(flag==1) {

      ang12=-(x2-x1)/(y2-y1);
      ang13=-(x3-x1)/(y3-y1);

      xp=(ang12*x12-ang13*x13-y12+y13)/(ang12-ang13);

      yp=ang12*(xp-x12)+y12;
    
    }

    r=sqrt((xp-x1)*(xp-x1)+(yp-y1)*(yp-y1));

    printf("%.3lf %.3lf %.3lf\n",xp,yp,r);
  }

  return 0;
  
}

C++: #include <iostream>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <algorithm>
#include <stdlib.h>

using namespace std;
double a, b, c, d, e, f;
double x, y;


void function(){
	if(c/b == f/e){
		x = 0;
		y = c/b;
	}else if(c/a == f/d){
		y = 0;
		x = c/a;
	}else{
		y = (c*d - a*f)/(d*b - a*e);
		x = (c - b*y)/a;
	}
	return;
}


int main(){
	int n;
	cin >> n;

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

		if((y2-y1) == 0){
			a = 2*(x3 - x1);
			b = 2*(y3 - y1);
			c = (x3 - x1)*(x1 + x3) + (y3 - y1)*(y1 + y3);

			d = 2*(x3 - x2);
			e = 2*(y3 - y2);
			f = (x3 - x2)*(x2 + x3) + (y3 - y2)*(y2 + y3);
		}else if((y3-y1) == 0){

			a = 2*(x3 - x2);
			b = 2*(y3 - y2);
			c = (x3 - x2)*(x2 + x3) + (y3 - y2)*(y2 + y3);

			d = 2*(x2 - x1);
			e = 2*(y2 - y1);
			f = (x2 - x1)*(x1 + x2) + (y2 - y1)*(y1 + y2);
		}else{

			a = 2*(x3 - x1);
			b = 2*(y3 - y1);
			c = (x3 - x1)*(x1 + x3) + (y3 - y1)*(y1 + y3);

			d = 2*(x2 - x1);
			e = 2*(y2 - y1);
			f = (x2 - x1)*(x1 + x2) + (y2 - y1)*(y1 + y2);
		}
		function();
		double r;
		r = sqrt((x - x1)*(x - x1) + (y - y1)*(y - y1));

		printf("%.3lf %.3lf %.3lf",x, y, r);
	}
	return 0;
}