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 <stdio.h>
#define _USE_MATH_DEFINES
#include <math.h>

int main(void){
	int n , i;
	float x1,x2,x3,y1,y2,y3,a1,a2,b1,b2,c1,c2,xp,yp,r;
	scanf("%d" , &n);
	for(i=1;i<=n;i++){
		scanf("%f %f %f %f %f %f" , &x1 , &y1 , &x2 , &y2 , &x3 , &y3);
		a1 = 2 * (x2 - x1);
		b1 = 2 * (y2 - y1);
		c1 = x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2;
		a2 = 2 * (x3 - y1);
		b2 = 2 * (y3 - y1);
		c2 = x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3;
		xp = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
		yp = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
		r = sqrt((xp - x1) * (xp - x1) + (yp - y1) * (yp - y1));
		printf("%.3f %.3f %.3f\n" , xp+0.0 , yp+0.0 , r+0.0);
	}
	return 0;
}

C++: #include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
#define all(c) (c).begin(),(c).end()
#define pb push_back
#define mp make_pair
#define ERASE(v,i) (v).erase(remove(all(v),i),(v).end())
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(it,c) for(typeof((c).begin()) it=(c).begin();it!=(c).end();++it)
#define debug(x) cerr<<#x<<" = "<<(x)<<endl;
#define LINE cerr<<"LINE: "<<__LINE__<<endl;
inline int toInt(string s){int v;istringstream i(s);i>>v;return v;}
template<class T> inline string toString(T x){ostringstream o;o<<x;return o.str();}
template<class T> void pv(T a,T b){for(T it=a;it!=b;++it)cerr<<*it<<" ";cerr<<endl;}
const int INF = 1000000000;
const double PI = acos(-1.0), EPS = 1e-10;

inline double SQ(double x) {
	return x * x;
}

inline double DIST(double x1, double y1, double x2, double y2) {
	return sqrt(SQ(x1 - x2) + SQ(y1 - y2));
}

int main() {
	int N; cin >> N;
	while(N--) {
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		double A = x1 - x2;
		double B = y1 - y2;
		double C = x1 - x3;
		double D = y1 - y3;
		
		double S = SQ(x1) - SQ(x2) + SQ(y1) - SQ(y2);
		double T = SQ(x1) - SQ(x3) + SQ(y1) - SQ(y3);
		
		double delta = (A * D - B * C);
		
		double X = (D * S - B * T) / 2 / delta;
		double Y = (-1 * C * S + A * T) / 2 / delta;
		
		printf("%.3f %.3f %.3f\n", X, Y, DIST(x1, y1, X, Y));
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;

//line is ax + by = c
struct Line{
  double a, b, c;
};

struct Node{
  double x, y;
};

Line perpendicular_bisector(Node a, Node b){
  Line result;
  result.a = a.x - b.x;
  result.b = a.y - b.y;
  result.c = (a.x * a.x - b.x * b.x + a.y * a.y - b.y * b.y) / 2;
  return result;
}

Node intersection(Line l, Line m){
  Node result;
  result.x = (m.b * l.c - l.b * m.c) / (l.a * m.b - m.a * l.b);
  result.y = (m.a * l.c - l.a * m.c) / (m.a * l.b - l.a * m.b);
  return result;
}

double distance(Node a, Node b){
  return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}

void solve(){
  Node A, B, C;
  cin >> A.x >> A.y;
  cin >> B.x >> B.y;
  cin >> C.x >> C.y;
  Line l = perpendicular_bisector(A, B);
  Line m = perpendicular_bisector(A, C);
  Node center = intersection(l, m);
  double raidus = distance(center, A);
  printf("%.3lf %.3lf %.3lf", center.x, center.y, raidus);
}

int main(){
  int N;
  cin >> N;
  while(N--){
    solve();
  }
}