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 <cstdio>

using namespace std;

double sisya(double a) {
	a *= 1000.0;
	if (a-(int)a >= 0.5) {
		a += 1.0;
	}
	a /= 1000.0;
	return a; 
}

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, b, c;
		a = sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2));
		b = sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
		c = sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
		double s = (a+b+c)/2.0;
		double S = sqrt(s*(s-a)*(s-b)*(s-c));
		double p1, p2, pr;
		p1 = (a*a*(b*b+c*c-a*a)*x1+b*b*(c*c+a*a-b*b)*x2+c*c*(a*a+b*b-c*c)*x3)/(16.0*S*S);
		p2 = (a*a*(b*b+c*c-a*a)*y1+b*b*(c*c+a*a-b*b)*y2+c*c*(a*a+b*b-c*c)*y3)/(16.0*S*S);
		pr = (a*b*c)/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
		p1 = sisya(p1);
		p2 = sisya(p2);
		pr = sisya(pr);
		printf("%.3f %.3f %.3f\n", p1, p2, pr);
	}
	return 0;
}

C++: #include <cmath>
#include <cstdio>
#include <iostream>

using namespace std;

struct point {
	double x, y;
}A, B, C;

struct circle {
	double r;
	point cen;
}cir;

//求三角形外接&#22278;&#22278;心坐&#26631;及半径
//&#36755;入三角形三点坐&#26631; 
void solve(point A, point B, point C) {
	double tmp1 = A.x * A.x - B.x * B.x + A.y * A.y - B.y * B.y;
	double tmp2 = A.x * A.x - C.x * C.x + A.y * A.y - C.y * C.y;   //外接&#22278;&#22278;心坐&#26631;公式 
	cir.cen.x = (tmp1 * (A.y - C.y) - tmp2 * (A.y - B.y)) / (2 * (A.y - C.y) * (A.x - B.x) - 2 * (A.y - B.y) * (A.x - C.x));
	cir.cen.y = (tmp1 * (A.x - C.x) - tmp2 * (A.x - B.x)) / (2 * (A.y - B.y) * (A.x - C.x) - 2 * (A.y - C.y) * (A.x - B.x));
	double la = sqrt((A.x - B.x) * (A.x - B.x) + (A.y - B.y) * (A.y - B.y)); //三角形三&#36793;&#38271; 
	double lb = sqrt((B.x - C.x) * (B.x - C.x) + (B.y - C.y) * (B.y - C.y));
	double lc = sqrt((C.x - A.x) * (C.x - A.x) + (C.y - A.y) * (C.y - A.y));
	double p = (la + lb + lc) / 2.0;
	double s = sqrt(p * (p - la) * (p - lb) * (p - lc)); //三角形面&#31215; 
	cir.r = la * lb * lc / (4 * s); //外接&#22278;半径公式 
}

int main() {
	int t;
	scanf ("%d", &t);
	while (t--) {
		scanf ("%lf%lf%lf%lf%lf%lf", &A.x, &A.y, &B.x, &B.y, &C.x, &C.y);
		solve(A, B, C);
		printf ("%.3lf %.3lf %.3lf\n",cir.cen.x, cir.cen.y, cir.r);
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define int long long
#define rep(i,n) for(int i=0;i<n;i++)
#define Rep(i,a,b) for(int i=a;i<b;i++)
#define REP(i,a,b) for(int i=a;i<=b;i++)
#define rev(i,n) for(int i=n-1;i>=0;i--)
#define vi vector<int>
#define vvi vector<vi>
#define pb push_back
#define pi pair<int,int>
#define vp vector<pair<int,int>>
#define mp make_pair
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
#define MEMSET(a) memset(a,0,sizeof(a))
#define inf (1ll<<60)
#define Yes(f) cout<<(f?"Yes":"No")<<endl
#define yes(f) cout<<(f?"yes":"no")<<endl
#define YES(f) cout<<(f?"YES":"NO")<<endl
#define SORT(v) sort(all(v))
#define RSORT(v) sort(all(v), greater<int>())
#define VI(a,n) vi a(n); rep(i,n)cin>>a[i];
#define sq(n) (n)*(n)

using namespace std;

const int mod=1e9+7;

void run();

void init() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout<<fixed<<setprecision(3);
}

signed main(){
    init();
    run();
    return 0;
}

void run(){
    int n;
    cin>>n;
    while(n--){
        double x1,x2,x3,y1,y2,y3;
        cin>>x1>>y1>>x2>>y2>>x3>>y3;
        double a=2*(x2-x1),b=2*(y2-y1),c=sq(x2)-sq(x1)+sq(y2)-sq(y1),d=2*(x3-x2),e=2*(y3-y2),f=sq(x3)-sq(x2)+sq(y3)-sq(y2);
        double y=(c*d-a*f)/(b*d-a*e);
        double x=(c-b*y)/a;
        if(x==-0)x=0;
        if(y==-0)y=0;
        double r=sqrt(sq(x-x1)+sq(y-y1));
        cout<<x<<" "<<y<<" "<<r<<endl;
    }
}