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 <iomanip>

using namespace std;

void getSide(double x1, double y1, double x2, double y2, double x3, double y3, double *a, double *b, double *c) {
    double marginX1, marginX2, marginX3;
    double marginY1, marginY2, marginY3;
    
    marginX1 = fabs(x1 - x2);
    marginX2 = fabs(x2 - x3);
    marginX3 = fabs(x3 - x1);
    marginY1 = fabs(y1 - y2);
    marginY2 = fabs(y2 - y3);
    marginY3 = fabs(y3 - y1);
    
    *a = sqrt(pow(marginX1, 2) + pow(marginY1, 2));
    *b = sqrt(pow(marginX2, 2) + pow(marginY2, 2));
    *c = sqrt(pow(marginX3, 2) + pow(marginY3, 2));
    
}

    // 半径取得の処理
double getR(double a, double b, double c) {
    double r, s;
    s = (a + b + c) / 2;
    r = (a * b * c) / (4 * sqrt(s * (s - a) * (s - b) * (s - c)));
    return r;
}

    // 中心座標を求める処理
void getCentralCoodinate(double x1, double y1, double x2, double y2, double x3, double y3, double *xp, double *yp) {
    double a1, b1, c1;
    double a2, b2, c2;
    double marginX1, marginX3;
    double marginY1, marginY3;
    
    *xp = 0;
    *yp = 0;
    
    marginX1 = fabs(x1 - x2);
    marginX3 = fabs(x3 - x1);
    marginY1 = fabs(y1 - y2);
    marginY3 = fabs(y3 - y1);
    
    a1 = 2 * marginX1;
    b1 = 2 * marginY1;
    a2 = 2 * marginX3;
    b2 = 2 * marginY3;
    
    c1 = pow(x1, 2) - pow(x2, 2) + pow(y1, 2) - pow(y2, 2);
    c2 = pow(x1, 2) - pow(x3, 2) + pow(y1, 2) - pow(y3, 2);
    
    *xp = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
    *yp = (a2 * c1 - a1 * c2) / (a1 * b2 - a2 * b1);
}

    // main処理
int main(void) {
    double x1, x2, x3;
    double y1, y2, y3;
    double xp, yp;
    double a, b, c, r;
    int n;
    
    cin >> n;
    while(n--) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        getSide(x1, y1, x2, y2, x3, y3, &a, &b, &c);
        r = getR(a, b, c);
        getCentralCoodinate(x1, y1 ,x2, y2, x3, y3, &xp, &yp);
        
        cout << setprecision(3);
        cout << setiosflags(ios::fixed);
        cout << xp << " " << yp << " " << r << endl;
    }
    
    return 0;
}

C++: 
// Circumscribed Circle of a Triangle


#include<stdio.h>
#include<iostream>
#include<math.h>
#include<iomanip>
using namespace std;

int main()
{
	int n;
	cin >> n;
	double x[3];
	double y[3];
	double r;

	while(n--){
	 for(int j=0;j<3;j++){
	 	cin >> x[j] >> y[j];
	 }

	 double a = 2*(x[1]-x[0]);
	 double b = 2*(y[1]-y[0]);
	 double c = 2*(x[2]-x[0]);
	 double d = 2*(y[2]-y[0]);

	 double e = pow(x[1],2.0)-pow(x[0],2.0)+pow(y[1],2.0)-pow(y[0],2.0);
	 double f = pow(x[2],2.0)-pow(x[0],2.0)+pow(y[2],2.0)-pow(y[0],2.0);

	 if(a==0){
	 	swap(a,c);
	 	swap(b,d);
	 	swap(e,f);
	 }

	b = b/a;
	e = e/a;
	a = a/a;
	 
	 
	d = d-b*c;
	f = f-e*c;
	c = c-a*c;

	f = f/d;
	d = d/d;

	b = b-d*b;
	e = e-f*b;

	r = sqrt(pow((e-x[0]),2.0)+pow((f-y[0]),2.0));
	
	cout << fixed;
	cout << setprecision(3) << e <<" "<< f << " " << r << endl;
	}

}

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

using namespace std;

int main(void){
	int n, i, j;
	double x1,y1,x2,y2,x3,y3, r, px, py;
	scanf("%d",&n);
	for(i = 0;i < n;i++){
		scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
		if((y2 - y1) * (x3 - x1) - (y3 - y1) * (x2 - x1) == 0){
			if(x1 == x2 && x2 == x3){
				px = x1;
				py = (max(y1,max(y2,y3)) + min(y1,min(y2,y3))) / 2;
				r = fabs(max(y1,max(y2,y3)) - py);
			}
			else{
				double tx1, ty1, tx2, ty2;
				if(x1 > x2 && x1 > x3){
					tx1 = x1, ty1 = y1;
				}else if(x2 > x1 && x2 > x3){
					tx1 = x2, ty1 = y2;
				}else{
					tx1 = x3, ty1 = y3;
				}
				if(x1 < x2 && x1 < x3){
					tx2 = x1, ty2 = y1;
				}else if(x2 < x1 && x2 < x3){
					tx2 = x2, ty2 = y2;
				}else{
					tx2 = x3, ty2 = y3;
				}
				px = (tx1 + tx2) / 2;
				py = (ty1 + ty2) / 2;
				r = sqrt(pow(tx1 - px,2.0) + pow(ty1 - py,2.0));
			}
		}else{
			py = ((x3 - x1) * (pow(x2,2.0) - pow(x1,2.0) + pow(y2,2.0) - pow(y1,2.0)) - (x2 - x1) * (pow(x3,2.0) - pow(x1,2.0) + pow(y3,2.0) - pow(y1,2.0))) / (2 * ((y2 - y1) * (x3 - x1) - (y3 - y1) * (x2 - x1)));
			px = ((y3 - y1) * (pow(y2,2.0) - pow(y1,2.0) + pow(x2,2.0) - pow(x1,2.0)) - (y2 - y1) * (pow(y3,2.0) - pow(y1,2.0) + pow(x3,2.0) - pow(x1,2.0))) / (2 * ((x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)));
			r = sqrt(pow(px - x1,2.0) + pow(py - y1,2.0));
		}
		printf("%.3f %.3f %.3f\n",px,py,r);
	}
	return 0;
}