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++: #define _USE_MATH_DEFINES

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <map>

using namespace std;

typedef pair<long long int, long long int> P;
long long int INF = 1e18;

int main()
{
    const double EPS = 1.0e-10;
    double a,b,c,d,e,f;
    double x1,x2,x3,y1,y2,y3;
    double x,y,r;
    int n;
	cin >> n;
    for(int i = 0; i < n; i++){
    	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        a = x1 - x2;
        b = y1 - y2;
        c = (x1 * x1 + y1 * y1 - x2 * x2 - y2 * y2) / 2;
        d = x1 - x3;
        e = y1 - y3;
        f = (x1 * x1 + y1 * y1 - x3 * x3 - y3 * y3) / 2;
        x = (e * c - b * f) / (a * e - b * d);
        y = (a * f - d * c) / (a * e - b * d);
    	r = sqrt((x1 - x) * (x1 - x) + (y1 - y) * (y1 - y));
        if(-0.0005 < x && x < 0.0+EPS) x = 0.0;
        if(-0.0005 < y && y < 0.0+EPS) y = 0.0;
 
        printf("%.3f %.3f %.3f\n",x,y,r);
    }
    return 0;
}

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

double length(double x1, double y1, double x2, double y2) {
    return sqrt(pow(x2 - x1, 2) + pow(y2 - y1, 2));
}

double round(double d) {
    if (d < 0) {
        d *= 1000;
        d -= 0.5;
        int i = d;
        d = i / 1000.0;
    } else {
        d *= 1000;
        d += 0.5;
        int i = d;
        d = i / 1000.0;
    }

    return d;
}

int main() {

    int n;
    cin >> n;

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

        px = ((y1 - y3) * (pow(x1, 2) - pow(x2, 2) + pow(y1, 2) - pow(y2, 2)) - (y1 - y2) * (pow(x1, 2) - pow(x3, 2) + pow(y1, 2) - pow(y3, 2))) / ((x1 - x2) * (y1 - y3) - (x1 - x3) * (y1 - y2)) / 2.0;
        py = ((x1 - x3) * (pow(x1, 2) - pow(x2, 2) + pow(y1, 2) - pow(y2, 2)) - (x1 - x2) * (pow(x1, 2) - pow(x3, 2) + pow(y1, 2) - pow(y3, 2))) / ((x1 - x3) * (y1 - y2) - (x1 - x2) * (y1 - y3)) / 2.0;
        r = length(x1, x2, px, py);

        px = round(px);
        py = round(py);
        r = round(r);

        cout << fixed << setprecision(3) << px << " " << py << " " << r << endl;
    }

    return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<iostream>
#include<cstdlib>
#include<cmath>
using namespace std;
double GetLen(double x1,double y1,double x2,double y2){
  return sqrt((y2-y1)*(y2-y1)+(x2-x1)*(x2-x1));
}
main(){
  int n;
  double x1,y1,x2,y2,x3,y3; 
  double R,rx,ry;
  double a,b,c,a1,b1,c1,a2,b2,c2;
  scanf("%d",&n);
  for(int i=0;i<n;i++){
    scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);
    a=GetLen(x1,y1,x2,y2);
    b=GetLen(x2,y2,x3,y3);
    c=GetLen(x3,y3,x1,y1);
    R=(a*b*c)/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
    a1=2*(x2-x1);
    b1=2*(y2-y1);
    c1=x1*x1-x2*x2+y1*y1-y2*y2;
    a2=2*(x3-x1);
    b2=2*(y3-y1);
    c2=x1*x1-x3*x3+y1*y1-y3*y3;
    rx=(b1*c2-b2*c1)/(a1*b2-a2*b1);
    ry=(c1*a2-c2*a1)/(a1*b2-a2*b1);
    printf("%.3f %.3f %.3f\n",rx,ry,R);
  }
}