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 <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for(int i = s; i < (int)n; i++)
#define per(i, n) for(int i = n; i >= 0; i--)
#define ROF(i, s, n) for(int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n) for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define CLR(a) memset((a), 0 ,sizeof(a))
#define NCLR(a) memset((a), -1 ,sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\n";
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << "\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue< int, vector<int>, greater<int> > minpq;
static const double EPS = 1e-10;
static const double PI = acos( -1.0 );
static const int mod = 1000000007;
static const int INF = 1 << 25;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = { -1, 0, 1, 0, 1, -1, 1, -1 };
static const int dy[] = { 0, -1, 0, 1, 1, 1, -1, -1 };

int n;
double x_1, y_1, x_2, y_2, x_3, y_3, x_4, y_4;

int main() {
	scanf( "%d", &n );
	for ( int i = 0; i < n; i++ ) {
		scanf( "%lf %lf %lf %lf %lf %lf", &x_1, &y_1, &x_2, &y_2, &x_3, &y_3 );
		double a1 = 2 * ( x_2 - x_1 );
		double b1 = 2 * ( y_2 - y_1 );
		double c1 = sq( x_1 ) - sq( x_2 ) + sq( y_1 ) - sq( y_2 );
		double a2 = 2 * ( x_3 - x_1 );
		double b2 = 2 * ( y_3 - y_1 );
		double c2 = sq( x_1 ) - sq( x_3 ) + sq( y_1 ) - sq( y_3 );
		double x = ( b1 * c2 - b2 * c1 ) / ( a1 * b2 - a2 * b1 );
		double y = ( c1 * a2 - c2 * a1 ) / ( a1 * b2 - a2 * b1 );

		double r = sqrt( sq( x - x_1 ) + sq( y - y_1 ) );
		printf( "%.3f %.3f %.3f\n", x, y, r );
	}
	return 0;
}

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

float a, b, c, d, e, f;
float x, y;

void solve(){
  float tmp1 = f-(c*d/a);
  float tmp2 = e-(b*d/a);
  y = tmp1/tmp2;
  x = (c - b * y) / a;
  printf("%.3f %.3f ", x, y);        
  //printf("y=%f\n", (f-(c*d/a))/(e-(b*d/a)));
  // ツ四ツ偲個古慊禿シ
  int tx = (int)(10000 * x);
  if (tx % 10 >= 5) {
    tx += 10;
  }
  tx /= 10;
  x = (float)tx / 1000.0f;

  int ty = (int)(10000 * y);
  if (ty % 10 >= 5) {
    ty += 10;
  }
  ty /= 10;
  y = (float)ty / 1000.0f;
}

int main(int argc, char const* argv[]) {
  int n;
  cin>>n;
  for (int i = 0; i < n; i += 1) {
    float x1, y1, x2, y2, x3, y3;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    a = -2*x1 + 2*x2;
    b = -2*y1 + 2*y2;
    c = x2*x2 + y2*y2 - x1*x1 - y1*y1;
    d = -2*x1 + 2*x3;
    e = -2*y1 + 2*y3;
    f = x3*x3 + y3*y3 - x1*x1 - y1*y1;

    solve();
    
    float r = (x - x1)*(x - x1) + (y - y1)*(y - y1);
    r = sqrt(r);
    int tr = (int)(10000 * r);
    if (tr % 10 >= 5) {
      tr += 10;
    }
    tr /= 10;
    r = (float)tr / 1000.0f;
    
    printf("%.3f\n", r);
  }  
  return 0;
}