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;

    // 半径取得の処理
double getR(double x1, double y1, double x2, double y2, double x3, double y3) {
    
    double marginX1, marginX2, marginX3;
    double marginY1, marginY2, marginY3;
    double a, b, c;
    double r, s;
    
        // それぞれのxとyの長さを求める
    marginX1 = abs(x2 - x1);
    marginY1 = abs(y2 - y1);
    marginX2 = abs(x1 - x3);
    marginY2 = abs(y1 - y3);
    marginX3 = abs(x3 - x2);
    marginY3 = abs(y3 - y2);
    
        // それぞれの辺の長さを求める
    a = sqrt(pow(marginX1,2) + pow(marginY1,2));
    b = sqrt(pow(marginX2,2) + pow(marginY2,2));
    c = sqrt(pow(marginX3,2) + pow(marginY3,2));
    
        // 半径を求める
    s = (a + b + c) / 2;
    r = (a * b * c) / (4 * sqrt(s * (s - a) * (s - b) * (s - c)));
    
    r *= 10000;
    r = (int)r;
    r /= 10;
    r = round(r);
    r /= 1000;
    
    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 marginX1, marginX2;
    double marginY1, marginY2;
    double marginXp1, marginXp2;
    double marginYp1, marginYp2;
    double n, m;
    
        // 初期化
    *xp = 0;
    *yp = 0;
    
        // それぞれのxとyの長さを求める
    marginX1 = abs(x2 - x1);
    marginY1 = abs(y2 - y1);
    marginX2 = abs(x1 - x3);
    marginY2 = abs(y1 - y3);
    
        // 中心座標を求めるのに必要な式
    marginXp1 = 2 * marginX1;
    marginYp1 = 2 * marginY1;
    marginXp2 = 2 * marginX2;
    marginYp2 = 2 * marginY2;
    
    n = pow(x1,2) - pow(x2,2) + pow(y1,2) - pow(y2,2);
    m = pow(x1,2) - pow(x3,2) + pow(y1,2) - pow(y3,2);
    
    *xp = (marginYp1 * m - marginYp2 * n) / (marginXp1 * marginYp2 - marginXp2 * marginYp1);
    *yp = (marginXp2 * n - marginXp1 * m) / (marginXp1 * marginYp2 - marginXp2 * marginYp1);
    
    *xp *= 10000;
    *xp = (int)*xp;
    *xp /= 10;
    *xp = round(*xp);
    *xp /= 1000;
    
    *yp *= 10000;
    *yp = (int)*yp;
    *yp /= 10;
    *yp = round(*yp);
    *yp /= 1000;
}

    // main処理
int main(void) {
    double x1, x2, x3;
    double y1, y2, y3;
    double xp, yp;
    double r;
    int n;
    
    cin >> n;
    
    while(cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
        
            // 半径と中心座標を取得する処理
        r = getR(x1, y1, x2, y2, x3, y3);
        getCentralCoodinate(x1, y1, x2, y2, x3, y3, &xp, &yp);
        
            // 出力
        cout << showpoint << setprecision(4);
        cout << xp << " " << yp << " " << r << endl;
    }
    
    return 0;
}

C++: #include <stdio.h>
#include <complex>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef complex<double> P;

double cross(const P& a, const P& b) { return imag(conj(a)*b); }
P rot90(const P& p) { return p*P(0, 1); }
P crosspoint(const P& l0, const P& l1, const P& m0, const P& m1) {
    const double A = cross(l1-l0, m1-m0);
    const double B = cross(l1-l0, l1-m0);
    return m0 + B/A * (m1-m0);
}

P center(P a, P b, P c) {
    const P m1 = 0.5*(a+b), m2 = 0.5*(b+c);
    return crosspoint(m1, m1+rot90(b-a), m2, m2+rot90(c-b));
}

int main() {
    int T;
    scanf("%d", &T);
    while (T--) {
        double x1, y1, x2, y2, x3, y3;
        scanf("%lf%lf", &x1, &y1);
        scanf("%lf%lf", &x2, &y2);
        scanf("%lf%lf", &x3, &y3);
        const P p = center(P(x1, y1), P(x2, y2), P(x3, y3));
        printf("%.3f %.3f %.3f\n", p.real(), p.imag(), abs(p-P(x1, y1)));
    }
    return 0;
}

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

int main(void)
{
	float x[3][2];
	int n;
	int xa, xb, xc;
	int ya, yb, yc;
	float ans[3];
	int i, j;
	
	scanf("%d", &n);
	
	while (n){
		for (i = 0; i < 3; i++){
			for (j = 0; j < 2; j++){
				scanf("%f", &x[i][j]);
			}
		}
		
		xa = 2 * (x[1][0] - x[0][0]);
		xb = 2 * (x[1][1] - x[0][1]);
		xc = x[0][0] * x[0][0] - x[1][0] * x[1][0] + x[0][1] * x[0][1] - x[1][1] * x[1][1];
		ya = 2 * (x[2][0] - x[0][0]);
		yb = 2 * (x[2][1] - x[0][1]);
		yc = x[0][0] * x[0][0] - x[2][0] * x[2][0] + x[0][1] * x[0][1] - x[2][1] * x[2][1];
		
		ans[0] = (xb * yc - yb * xc) / (xa * yb - ya * xb);
		ans[1] = (xc * ya - yc * xa) / (xa * yb - ya * xb);
		ans[2] = sqrt((ans[0] - x[0][0]) * (ans[0] - x[0][0]) + (ans[1] - x[0][1]) * (ans[1] - x[0][1]));
		
		for (i = 0; i < 3; i++){
			if (i){
				printf(" ");
			}
			printf("%.3f", ans[i]);
		}
		printf("\n");
		
		n--;
	}
	
	return (0);
}