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<cstdio>
#include<cmath>
using namespace std;

int main(){
	double x[3], y[3], n, xp, yp, r;
	scanf("%d", &n );
	for( int i=0; i<n; i++ ){
		scanf("%lf %lf %lf %lf %lf %lf", &x[0], &y[0], &x[1], &y[1], &x[2], &y[2]);
		double A1 = 2.0 * (x[1]-x[0]);
		double B1 = 2.0 * (y[1]-y[0]);
		double C1 = x[0]*x[0] - x[1]*x[1] + y[0]*y[0] - y[1]*y[1];
		double A2 = 2.0 * (x[2]-x[0]);
		double B2 = 2.0 * (y[2]-y[0]);
		double C2 = x[0]*x[0] - x[2]*x[2] + y[0]*y[0] - y[2]*y[2];
		xp = (B1*C2-B2*C1)/(A1*B2-A2*B1);
		yp = (C1*A2-C2*A1)/(A1*B2-A2*B1);
		r = sqrt( (xp-x[0])*(xp-x[0])+(yp-y[0])*(yp-y[0]) );
		printf("%.3f %.3f %.3f\n", xp, yp, r);
	}
	return 0;
}

C++: # include <iostream>
# include <algorithm>
# include <vector>
# include <string>
# include <set>
# include <map>
# include <cmath>
# include <iomanip>
# include <functional>
# include <utility>
# include <stack>
# include <queue>
# include <list>
using namespace std;
using LL = long long;
constexpr int MOD = 1000000000 + 7;
//?¶????????????????????????????????????§??????????????????????¢????????????????
constexpr int INF = 2000000000;
const double PI = acos(-1);

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 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 = (f - e*y) / d;
		cout << x << " " << y << hypot(x1 - x, y1 - y) << endl;
	}
}

C++: 
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <stack>
#include <cctype>
#include <complex>
#include <vector>
#include <algorithm>

using namespace std;


bool solve(){
	complex<double> Ans;
	complex<double> P[3];
	for(int i=0;i<3;i++){
		cin>> P[i].real()>> P[i].imag();
	}
	complex<double> T[2];
	complex<double> K[2];
	double katamuki[2];
	for(int i=0;i<2;i++){
		T[i].real() = (P[i].real() + P[i+1].real()) / 2;
		T[i].imag() = (P[i].imag() + P[i+1].imag()) / 2;
		complex<double> N;
		N = (P[i+1]-P[i])/abs(P[i+1]-P[i]);
		K[i].real() =  N.imag();
		K[i].imag() = -N.real();
	}
	int a = (T[1]-T[0]).real();
	int b = (T[1]-T[0]).imag();
	int x0 = K[0].real();
	int y0 = K[0].imag();
	int x1 = K[1].real();
	int y1 = K[1].imag();
	int m = ( a * y0 - b * x0 ) / ( x0 * y1 - x1 * y0);
	K[1] *= m;
	Ans = K[1] + T[1];

	printf("%.3f %.3f %.3f", Ans.real(), Ans.imag(), abs(Ans-P[0]));
	printf("\n");
	
	return true;
}

int main(){
	cout.setf(ios::fixed);
	cout.precision(5);
	int n;
	cin>> n;
	for(int i=0;i<n;i++){
		solve();
	}
	return 0;
}

C++: #include <iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cctype>
#include<queue>
#include<stack>
#include<stdio.h>
#include<vector>
#include<set>
#include<map>
#include<iomanip>
#define rep(i,n) for(int i=0;i<n;i++)
typedef int long long ll;

using namespace std;
int main(){
    int n;
    cin>>n;
    for(int i=0;i<n;i++){
    double a,b,c,d,e,f;
    cin>>a>>b>>c>>d>>e>>f;
    double x,y;
    y=((e-a)*(a*a+b*b-c*c-d*d)-(c-a)*(a*a+b*b-e*e-f*f))/(2*(e-a)*(b-d)-2*(c-a)*(b-f));
    if(c-a!=0){
         x=(2*(b-d)*y-a*a-b*b+c*c+d*d)/(2*(c-a));
    }else{
        x=(2*(b-f)*y-a*a-b*b+e*e+f*f)/(2*(e-a));
    }
    double r=sqrt(pow(x-a,2)+pow(y-b,2));
    printf("%.3f %.3f %.3f",x,y,r);
    }
    
    
    return 0;
}

C++: #include <bits/stdc++.h>
#include <vector>
using namespace std;

double n, r, x1, y1, x2, y2, x3, y3, xa, ya, xb, yb, xc, yc, ma, mb, mc, y_hasil, x_hasil;

int main(){
	cin >> n;
	for(int i = 1; i <= n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		ma = (x2-x1)/(y1-y2);
		mb = (x3-x2)/(y2-y3);
		mc = (x3-x1)/(y1-y3);
		xa = (x1+x2)/2;
		ya = (y1+y2)/2;
		xb = (x2+x3)/2;
		yb = (y2+y3)/2;
		xc = (x3+x1)/2;
		yc = (y3+y1)/2;
		
	//	cout << ma << " " << xa << " " << mb << " " << xb << " " << yb << " " << ya << "\n";
		if(y1 - y2 == 0){
			x_hasil = (x2 + x1) / 2;
			y_hasil = mb*(x_hasil - xb) + yb;
		}else if(y2 - y3 == 0){
			x_hasil = (x2 + x3) / 2;
			y_hasil = ma*(x_hasil - xa) + ya;
		}else{
			x_hasil = (ma*xa - mb*xb + yb - ya)/(ma - mb);
			y_hasil = ma*(x_hasil - xa) + ya;
		}
		
		r = sqrt((x_hasil - x1)*(x_hasil - x1) + (y_hasil - y1)*(y_hasil - y1));
		
		cout << fixed << setprecision(3) << x_hasil << " " << y_hasil << " " << r << "\n";
	}
	return 0;
}