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>
#include<cstdio>
using namespace std;


int main(void){

  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;

    double a = sqrt(pow(x2-x1,2) + pow(y2-y1,2));
    double b = sqrt(pow(x3-x2,2) + pow(y3-y2,2));
    double c = sqrt(pow(x3-x1,2) + pow(y3-y1,2));

    double r = a*b*c / sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));

    double A = pow(x2-x1,2) + pow(y2-y1,2);
    double B = pow(x3-x2,2) + pow(y3-y2,2);
    double C = pow(x3-x1,2) + pow(y3-y1,2);
    double s = (A*(B+C-A) + B*(C+A-B) + C*(A+B-C));


    double x = A*(B+C-A) / s;
    double y = B*(C+A-B) / s;
    if(x == 0){
      x = 0;
    }
    if(y == 0){
      y=0;
    }

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

  }




  return 0;
}

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

/*
  P1(x1, y1) P2(x2, y2) P3(x3, y3) P0(x0, y0)

 <円の中心(x0, y0)>
  2*(x2-x1)*x0 + 2*(y2-y1)*y0 = x2*x2 - x1*x1 + y2*y2 - y1*y1
  2*(x3-x1)*x0 + 2*(y3-y1)*y0 = x3*x3 - x1*x1 + y3*y3 - y1*y1
  x0=((y3-y1)*(x2*x2-x1*x1+y2*y2-y1*y1)+(y1-y2)*(x3*x3-x1*x1+y3*y3-y1*y1))/(2*(x2-x1)*(y3-y1)-(y2-y1)*(x3-x1))
  y0=((x1-x3)*(x2*x2-x1*x1+y2*y2-y1*y1)+(x2-x1)*(x3*x3-x1*x1+y3*y3-y1*y1))/(2*(x2-x1)*(y3-y1)-(y2-y1)*(x3-x1))

 <円の半径R>
  R = a/(2*sqrt(q-cosA*cosA)) 
  a = sqrt( (x2-x3)*(x2-x3) + (y2-y3)*(y2-y3) ) // P2-P3
  b = sqrt( (x1-x3)*(x1-x3) + (y1-y3)*(y1-y3) ) // P1-P3
  c = sqrt( (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1) ) // P1-P2
  cosA = (b*b + c*c - a*a)/(2*b*c)
 */

int main()
{
    double x1, x2, x3, y1, y2, y3; // 3角形の3点
    double x0, y0; // 円の中心
    double R;
    double a, b, c, cosA;
    int n;
    cin >> n;

    for(int i=0; i<n; i++){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        x0=((y3-y1)*(x2*x2-x1*x1+y2*y2-y1*y1)+(y1-y2)*(x3*x3-x1*x1+y3*y3-y1*y1))/(2*((x2-x1)*(y3-y1)-(y2-y1)*(x3-x1)));
        y0=((x1-x3)*(x2*x2-x1*x1+y2*y2-y1*y1)+(x2-x1)*(x3*x3-x1*x1+y3*y3-y1*y1))/(2*((x2-x1)*(y3-y1)-(y2-y1)*(x3-x1)));
        a = sqrt( (x2-x3)*(x2-x3) + (y2-y3)*(y2-y3) );
        b = sqrt( (x1-x3)*(x1-x3) + (y1-y3)*(y1-y3) );
        c = sqrt( (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1) );
        cosA = (b*b + c*c - a*a)/(2*b*c);
        R = a/(2*sqrt(1-cosA*cosA));

        printf("%.3f %.3f %.3f\n", x0, y0, R);
    }

    return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <complex>
#include <cstdio>

using namespace std;

typedef complex<double> P;

//ヤバそう？
#define X real()
#define Y imag()


class Triangle{
public:
	P p1,p2,p3;

	//e1:p2-p3間の辺
	double e1,e2,e3;

	//a1:e1の向かいの角
	double a1,a2,a3;

	//cosine formula
	//長さaの辺の向かいの角の角度
	double cf(double a,double b,double c){
		return acos((b*b+c*c-a*a)/(2.0*b*c));
	}

	Triangle(P _p1,P _p2,P _p3){
		p1=_p1;
		p2=_p2;
		p3=_p3;

		e1=abs(p2-p3);
		e2=abs(p1-p3);
		e3=abs(p1-p2);

		a1=cf(e1,e2,e3);
		a2=cf(e2,e1,e3);
		a3=cf(e3,e1,e2);

	}

	P get_center(){
		return (sin(2*a1)*p1+sin(2*a2)*p2+sin(2*a3)*p3)/(sin(2*a1)+sin(2*a2)+sin(2*a3));
	}

	double get_r(){
		return e1/(2.0*sin(a1));
	}


};

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

	for(int i=0;i<n;i++){
		P p[3];
		for(int j=0;j<3;j++){
			double x,y;
			cin>>x>>y;
			p[j]=P(x,y);
		}
		Triangle T(p[0],p[1],p[2]);

		P xy=T.get_center();
		double r=T.get_r();

		printf("%.3f %.3f %.3f\n",xy.X,xy.Y,r);
	}


	return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i,n)for(int i=0;i<(n);i++)
using namespace std;

int main() {
	int n; scanf("%d", &n);
	rep(i, n) {
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double a = -x1 + x2, b = -y1 + y2, c = (-pow(x1, 2) - pow(y1, 2) + pow(x2, 2) + pow(y2, 2)) / 2;
		double d = -x1 + x3, e = -y1 + y3, f = (-pow(x1, 2) - pow(y1, 2) + pow(x3, 2) + pow(y3, 2)) / 2;
		double y = (c*d - a*f) / (b*d - a*e);
		double x = (c - b*y) / a;
		printf("%.3lf %.3lf %.3lf\n", x, y, hypot(x1 - x, y1 - y));
	}
}