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>
#include <math.h>

int main () {
	int n;
	float x1, x2, x3, y1, y2, y3;
	float m1, m2, b1, b2;
	float x, y, r, hx, hy;
	scanf("%d", &n);
	while (n--) {
		scanf("%f %f %f %f %f %f" , &x1, &y1, &x2, &y2, &x3, &y3);
		if (y1 == y2) {
			x = (x1 + x2) / 2.0;
			m2 = (y3 == y1) ? 0 : (y3 - y1) / (x3 - x1);
			hx = (x1 + x3) / 2.0;
			hy = (y1 + y3) / 2.0;
			b2 = m2 * hx + hy;
			y = -m2 * x + b2;
		} else if (y1 == y3) {
			x = (x1 + x3) / 2.0;
			m1 = (y2 == y1) ? 0 : (y2 - y1) / (x2 - x1);
			hx = (x1 + x2) / 2.0;
			hy = (y1 + y2) / 2.0;
			b1 = m1 * hx + hy;
			y = -m1 * x + b1;
		} else {
			m1 = (x2 == x1) ? 0 : (y2 - y1) / (x2 - x1);
			m2 = (x3 == x1) ? 0 : (y3 - y1) / (x3 - x1);
			hx = (x1 + x2) / 2.0;
			hy = (y1 + y2) / 2.0;
			b1 = m1 * hx + hy;
			hx = (x1 + x3) / 2.0;
			hy = (y1 + y3) / 2.0;
			b2 = m2 * hx + hy;
			x = (b1 - b2) / (m1 - m2);
			y = -m1 * x + b1;
		}
		printf("%.3f %.3f %.3f\n", x, y, sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1)));
	}
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
int main(){
    int k;
	double x[3],y[3],a,b,c,d,e,f,l,m,n;
    cin>>k;
	while(k-->0){
        cin>>x[0]>>y[0]>>x[1]>>y[1]>>x[2]>>y[2];
        a=2*(x[1]-x[0]);
        b=2*(y[1]-y[0]);
        c=(x[1]*x[1])+(y[1]*y[1])-(x[0]*x[0])-(y[0]* y[0]);
        d=2*(x[2]- x[0]);
        e=2*(y[2]- y[0]);
        f=(x[2]*x[2])+(y[2]*y[2])-(x[0]*x[0])-(y[0]*y[0]);
        l=(c*e-f*b)/(a*e-d*b);
        m=(c*d-f*a)/(b*d-e*a);
        n=sqrt((l-x[0])*(l-x[0])+(m-y[0])*(m-y[0]));
        printf("%.3f %.3f %.3f\n",l,m,n);
    }
    return 0;
}

C++:  /*
Write a program which prints the central coordinate (px, py) and the radius r of a circumscribed circle of a triangle which is constructed by three points (x1, y1)(x2, y2)(x3, y3) on the plane surface.
 */

void solve(double a, double b, double c, double d, double e, double f) {
	double px,py,r;
	// ¼ñªüñÂÌð_ y-y1 = y2-y1 / x2-x1 * x-x1 -> y2-y1=a, x2-x1=b, y = a/b*x - a/b*x1-y1 -> y = -b/a*x + c
	double sl1 = -1*(c-a)/(d-b);			// (x1, y1)-(x2, y2)Ì¼ñªüÌX«
	double sl2 = -1*(e-c)/(f-d);			// (x2, y2)-(x3, y3)Ì¼ñªüÌX«
	double mid1x = (a+c)/2; double mid1y = (b+d)/2;
	double mid2x = (c+e)/2; double mid2y = (d+f)/2;
	double t1 = mid1y-sl1*mid1x;
	double t2 = mid2y-sl2*mid2x;
	// y = sl1*x - t1 Æ y = sl2*x - t2 ðA§B -> sl1*x - t1 = sl2*x - t2 -> (sl1-sl2)*x = t1 - t2
	px = (t1-t2)/(sl1-sl2);
	py = sl1*px - t1;
	r = sqr(px,py,a,b);
	printf("%4.3f %4.3f %4.3f",px,py,r);
}

double sqr(double a, double b, double c, double d) {
	// ñÂÌ_Ì£ðßé\bh
	double s = sqrt(pow(max(a,c)-min(a,c), 2) + pow(max(b,d)-min(b,d), 2));
	return s;
}

int main() {
	int n; cin >> n;
	double x1, y1, x2, y2, x3, y3;
	for (int i=0; i<n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		solve(x1,y1,x2,y2,x3,y3);
	}
	return 0;
}

C++: #include <cstdio>
#include <complex>
#include <vector>
#include <cmath>

using namespace std;

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)

typedef complex<double> P;
typedef vector<P> VP;
const double EPF = 1e-5;

#define X real()
#define Y imag()
#define EQ(x,y) (abs(x-y) < EPF)

double dot(P a, P b) {
  return (conj(a)*b).X;
}
double cross(P a, P b) {
  return (conj(a)*b).Y;
}
double distPP(P a, P b) {
  return sqrt(dot(a-b,a-b));
}
P crosspoint(P a, P b, P c, P d) {//ac & bd
  double d1 = cross(d-b,b-a);
  double d2 = cross(d-b,c-a);
  if(EQ(d1,0) & EQ(d2,0)) return a;
  if(EQ(d2,0)) return a;
  return a + (c-a)*d1/d2;
}
P normal_vector(P a) {
  P b(a.Y, a.X);
  return b;
}

double t_r(double a) {
  return round(a * 1000)*1.0/1000;
}

int main() {
  int n;
  scanf("%d", &n);
  REP(j, n) {
    VP a;
    REP(i, 3) {
      double x,y;
      scanf("%lf %lf", &x, &y);
      a.push_back(P(x,y));
    }
    P M = (a[0]+a[1])/(P)2;
    P N = (a[1]+a[2])/(P)2;
    P M_v = normal_vector(a[0]-a[1]);
    P N_v = normal_vector(a[1]-a[2]);  
    P o = crosspoint(M, N, M+M_v, N+N_v);
    
    printf("%.3lf %.3lf %.3lf\n", t_r(o.X), t_r(o.Y), t_r(distPP(o,a[0])));
    a.clear();
  }
  return 0;
}