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<algorithm>
using namespace std;
class Point {
public:
	double x, y;
};
double distance(Point x, Point y) {
	return pow(pow(fabs(x.x - y.x), 2) + pow(fabs(x.y - y.y), 2), 0.5);
}
int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		Point p[3], s;
		for (int i = 0; i < 3; i++) {
			cin >> p[i].x >> p[i].y;
		}
		double a, b, c, d, e, f;
		a = (-2)*p[0].x + 2 * p[1].x;
		b = (-2)*p[0].y + 2 * p[1].y;
		c = (-2)*p[0].x + 2 * p[2].x;
		d = (-2)*p[0].y + 2 * p[2].y;
		e = pow(p[0].x, 2) + pow(p[0].y, 2) - pow(p[1].x, 2) - pow(p[1].y, 2);
		f = pow(p[0].x, 2) + pow(p[0].y, 2) - pow(p[2].x, 2) - pow(p[2].y, 2);
		s.x = (b*f - d*e) / (a*d - b*c);
		s.y = (a*f - c*e) / (b*c - a*d);
		double k = distance(p[0], s);
		s.x = round(s.x * 1000) / 1000;
		s.y = round(s.y * 1000) / 1000;
		k = round(k * 1000) / 1000;
		cout << s.x << " " << s.y << " " << k << endl;
	}
	return 0;
}

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

int main()
{
  double eps = 10e-7;
  double x1,y1,x2,y2,x3,y3;
  int T;
  scanf("%d",&T);
  for(int i=0;i<T;i++){
  scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);
  double a = pow(x2-x3,2.0)+pow(y2-y3,2.0);
  double b = pow(x3-x1,2.0)+pow(y3-y1,2.0);
  double c = pow(x1-x2,2.0)+pow(y1-y2,2.0);
  double a2 = sqrt(pow(x2-x3,2.0)+pow(y2-y3,2.0));
  double b2 = sqrt(pow(x3-x1,2.0)+pow(y3-y1,2.0));
  double c2 = sqrt(pow(x1-x2,2.0)+pow(y1-y2,2.0));
  double s = (a2+b2+c2)/2.0;
  double S = sqrt(s*(s-a2)*(s-b2)*(s-c2));
  double px = (a*(b+c-a) * x1 + b*(c+a-b) * x2 + c*(a+b-c) * x3) / (16.0*pow(S,2.0));
  double py = (a*(b+c-a) * y1 + b*(c+a-b) * y2 + c*(a+b-c) * y3) / (16.0*pow(S,2.0));
  double r = a2*b2*c2/(4.0*S);
  px += 0.00005 + eps;
  py += 0.00005 + eps;
  r += 0.00005 + eps;
  printf("%.3f %.3f %.3f\n",px,py,r);
  }
}

C++: /*Filename: aoj0010.cpp
 *Author: Mushiyo
 */

#include <stdio.h>
#include <iostream>
#include <cmath>

using namespace std;

struct Point {
	double x;
	double y;
};

double countDist(Point p1, Point p2) {
	double xDist = abs(p1.x - p2.x);
	double yDist = abs(p1.y - p2.y);

	return sqrt(xDist * xDist + yDist * yDist);
}

// find the circumcenter by finding the intersection of the perpendicular bisectors.
Point getCircumcenter (Point p1, Point p2, Point p3) {
	Point bias = p1;

	// a, b & c for ax + by = c
	double a1 = p2.x - p1.x, b1 = p2.y - p1.y, c1 = (a1 * a1 + b1 * b1) / 2;
	double a2 = p3.x - p1.x, b2 = p3.y - p1.y, c2 = (a2 * a2 + b2 * b2) / 2;
	/* translate the origin to p1
	 * let the center of the circumscribed circle be (x, y)
	 * we can get the value of (x, y) by solve the following simultaneous equations to :
	 * a1 * x + b1 * y = c1 --(1)
	 * a2 * x + b2 * y = c2 --(2)
	 *
	 * 1. after rearrange eq.1, we have:
	 * y = (c1 - a1 * x) / b1
	 * plugging this into eq.2, we have:
	 * x = (b1 * c2 - b2 * c1) / (a2 * b1 - a1 * b2)
	 *
	 * 2. after rearrange eq.1, we have:
	 * x = (c1 - b1 * y) / a1
	 * plugging this into eq.2, we have:
	 * y = (a1 * c2 - a2 * c1) / (a1 * b2 - a2 * b1)
	 *   = (a2 * c1 - a1 * c2) / (a2 * b1 - a1 * b2)
	*/
	double d = a2 * b1 - a1 * b2;
	Point center;
	center.x = bias.x + (b1 * c2 - b2 * c1) / d;
	center.y = bias.y + (a2 * c1 - a1 * c2) / d;

	return center;
}

int main() {
	int n;

	while (cin >> n) {
		while (n > 0) {
			Point a, b, c;
			cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;

			Point center = getCircumcenter(a, b, c);
			double radius = countDist(center, a);

			printf("%.3f %.3f %.3f\n", center.x, center.y, radius);

			--n;
		}
	}

	return 0;
}