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 <vector>
#include<iomanip>
using namespace std;

struct Circles {
	double x;
	double y;
	double r;
};

Circles countCircle(double x1,double y1, double x2, double y2, double x3, double y3){
	double a = ((y2 - y1)*(y3*y3 - y1 * y1 + x3 * x3 - x1 * x1) - (y3 - y1)*(y2*y2 - y1 * y1 + x2 * x2 - x1 * x1)) / (2.0*((x3 - x1)*(y2 - y1) - (x2 - x1)*(y3 - y1)));
	double b = ((x2 - x1)*(x3*x3 - x1 * x1 + y3 * y3 - y1 * y1) - (x3 - x1)*(x2*x2 - x1 * x1 + y2 * y2 - y1 * y1)) / (2.0*((y3 - y1)*(x2 - x1) - (y2 - y1)*(x3 - x1)));
	double r = sqrt((x1 - a)*(x1 - a) + (y1 - b)*(y1 - b));
	return Circles{ a,b,r };
}


int main() {
	vector<Circles> dst;
	int n;
	double x1, y1, x2, y2, x3, y3;
	cin >> n;
	while (n-->0) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		dst.push_back(countCircle(x1, y1, x2, y2, x3, y3));
	}
	for (vector<Circles>::iterator it = dst.begin(); it < dst.end(); it++) {
		cout.setf(ios::fixed);
		cout << fixed << setprecision(3) << (*it).x << " " << (*it).y << " " << (*it).r << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <complex>
#include <vector>
#include <algorithm>
using namespace std;

typedef complex<double> P; /* ツ点 */

int main(){
	int n;
	cin>>n;
	double x1, y1, x2, y2, x3, y3;
	while(cin>>x1>>y1>>x2>>y2>>x3>>y3){
		double a1 = 2*(x2-x1);
		double b1 = 2*(y2-y1);
		double c1 = x1*x1-x2*x2+y1*y1-y2*y2;
		double a2 = 2*(x3-x1);
		double b2 = 2*(y3-y1);
		double c2 = x1*x1-x3*x3+y1*y1-y3*y3;
		double cx = (b1*c2-b2*c1)/(a1*b2-a2*b1);
		double cy = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		P cen2 = P(cx, cy);
		double r2 = abs(p1-cen2);

		printf("%.3f %.3f %.3f\n", real(cen2), imag(cen2), r2);
	}
	return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>
#include <vector>
using namespace std;
 
int main()
{
    int n;
    vector<double> x(3), y(3), x2(3), y2(3);
    cin >> n;
 
    for(int p=0; p<n; p++)
    {
        cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
        for(int i=0; i<3; i++) { x2[i] = x[i]*x[i]; y2[i] = y[i]*y[i]; }
 
        double a, b, c, a2, b2, c2;
        a2 = abs((x[2]-x[1])*(x[2]-x[1]) + (y[2]-y[1])*(y[2]-y[1]));
        b2 = abs((x[0]-x[2])*(x[0]-x[2]) + (y[0]-y[2])*(y[0]-y[2]));
        c2 = abs((x[1]-x[0])*(x[1]-x[0]) + (y[1]-y[0])*(y[1]-y[0]));
        a = sqrt(a2); b = sqrt(b2); c = sqrt(c2);
        double s, S2;
        s = (a + b + c) / 2;
        S2 = s * (s-a) * (s-b) * (s-c);
 
        double cx, cy;
        cx = (a2*(b2+c2-a2)*x[0] + b2*(c2+a2-b2)*x[1] + c2*(a2+b2-c2)*x[2]) / (16 * S2);
        cy = (a2*(b2+c2-a2)*y[0] + b2*(c2+a2-b2)*y[1] + c2*(a2+b2-c2)*y[2]) / (16 * S2);
        cout << setprecision(3) << fixed <<
            cx << " " << cy << " " << sqrt(abs((cx-x[0])*(cx-x[0])+(cy-y[0])*(cy-y[0]))) << endl;
    }
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <stdio.h>
using namespace std;
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;
    double x,y,r1,r2,r3;
    double a,b,c,d,e,f;
    a=2*x1-2*x2;
    b=2*y1-2*y2;
    c=x1*x1-x2*x2+y1*y1-y2*y2;
    d=2*x2-2*x3;
    e=2*y2-2*y3;
    f=x2*x2-x3*x3+y2*y2-y3*y3;
        x=(b*f-e*c)/(a*e-d*b);
        y=(c*d-f*a)/(a*e-d*b);
        
    r1=sqrt((x1-x)*(x1-x)+(y1-y)*(y1-y));
    r2=sqrt((x2-x)*(x2-x)+(y2-y)*(y2-y));
    r3=sqrt((x3-x)*(x3-x)+(y3-y)*(y3-y));
    
    printf("%.3f %.3f %.3f\n",x,y,r1);
}
}