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>
#include <string>
#include <sstream>

using namespace std;

long double sisya(long double a) {
	stringstream ss;
	string k;
	ss << a;
	ss >> k;
	if  (k.find('.') != string::npos) {
		int r = k.find('.');
		if (r+4 <= k.size() && k[r+4]-'0' >= 5) {
			a += 0.001;
		}
	}
	return a; 
}

double ll(double x1, double y1, double x2, double y2) {
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

double yoge(double a, double b, double c) {
	return (b*b+c*c-a*a)/(2.0*b*c);
}

int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		long double dd[3][2];
		for (int j = 0; j < 3; j++) {
			cin >> dd[j][0] >> dd[j][1];
		}
		long double s[3];
		s[0] = ll(dd[1][0],dd[1][1],dd[2][0],dd[2][1]);
		s[1] = ll(dd[2][0],dd[2][1],dd[0][0],dd[0][1]);
		s[2] = ll(dd[0][0],dd[0][1],dd[1][0],dd[1][1]);
		long double k[3];
		k[0] = acos(yoge(s[0],s[1],s[2]));
		k[1] = acos(yoge(s[1],s[2],s[0]));
		k[2] = acos(yoge(s[2],s[0],s[1]));
		long double X, Y, R;
		X = (dd[0][0]*sin(2.0*k[0])+dd[1][0]*sin(2.0*k[1])+dd[2][0]*sin(2.0*k[2]))/(sin(2.0*k[0])+sin(2.0*k[1])+sin(2.0*k[2]));
		Y = (dd[0][1]*sin(2.0*k[0])+dd[1][1]*sin(2.0*k[1])+dd[2][1]*sin(2.0*k[2]))/(sin(2.0*k[0])+sin(2.0*k[1])+sin(2.0*k[2]));
		R = s[0]/(2.0*sin(k[0]));
		X = sisya(X);
		Y = sisya(Y);
		R = sisya(R);
		printf("%.3f %.3f %.3f\n", X, Y, R);
	}
	return 0;
}

C++: #include <cmath>
#include <iostream>
#include <stdio.h>
using namespace std;

int main() {
  int n;
  double x1,y1,x2,y2,x3,y3,p,p1,p2,q,q1,q2,r;
  cin >> n;
  for(int i=0;i<n;i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    p1 = (x2 * x2 - x1 * x1 + y2 * y2 - y1 * y1) * (y3 - y1) - (x3 * x3 - x1 * x1 + y3 * y3 - y1 * y1) * (y2 - y1);
    p2 = 2 * (x1 * y2 - x1 * y3 + x2 * y3 - x2 * y1 + x3 * y1 - x3 * y2);
    p = p1 / p2;
    q1 = (y2 * y2 - y1 * y1 + x2 * x2 - x1 * x1) * (x3 - x1) - (y3 * y3 - y1 * y1 + x3 * x3 - x1 * x1) * (x2 - x1);
    q2 =  2 * (y1 * x2 - y1 * x3 + y2 * x3 - y2 * x1 + y3 * x1 - y3 * x2);
    q = q1 / q2;
    r = sqrt((x1-p) * (x1-p) + (y1-q) * (y1-q) );
    p *= 1000;
    p += 0.5;
    p = floor(p);
    p /= 1000;
    q *= 1000;
    q += 0.5;
    q = floor(q);
    q /= 1000;
    r *= 1000;
    r += 0.5;
    r = floor(r);
    r /= 1000;
    printf("%.3f %.3f %.3f", p, q, r);
  }
}

C++: #! -*- coding:utf-8 -*-
import math
n = input()
for x in xrange(n):
    x1,y1,x2,y2,x3,y3 = map(float,raw_input().split())
    a1 = 2.0*(x2-x1)
    b1 = 2.0*(y2-y1)
    x12 = x1**2
    y12 = y1**2
    c1 = x12-x2**2+y12+y2**2
    a2 = 2.0*(x3-x1)
    b2 = 2.0*(y3-y1)
    c2 = x12-x3**2+y12-y3**2
    denom=(a1*b2-a2*b1)
    x = (b1*c2-b2*c1)/denom
    y = (c1*a2-c2*a1)/denom
    r = math.sqrt((x-x1)**2+(y-y1)**2)

    print "%.3f %.3f %.3f"%(x,y,r)