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 <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <stdio.h>

#define MAX 1000000

using namespace std;

int main()
{
	double x1,y1,x2,y2,x3,y3,X1,Y1,X2,Y2,a1,b1,a2,b2;
	double xp,yp,r;
	int n,f;

	cin>>n;
	while(n--){
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		f=0;

		X1=(x1+x2)/2.0; Y1=(y1+y2)/2.0;
		X2=(x1+x3)/2.0; Y2=(y1+y3)/2.0;
	
		if(y2-y1!=0){
			if(x2!=x1)a1=-(y2-y1)/(x2-x1);
			else a1=0;
			b1=X1*(-a1)+Y1;
		}else f=1;

		if(y3-y1!=0){
			if(x3!=x1)a1=-(y3-y1)/(x3-x1);
			else a2=0;
			b2=X2*(-a2)+Y2;
		}else f=2;

		if(f==1){ 
			xp=X1;yp=a2*xp+b2;
		}else if(f==2){
			xp=X2; yp=a1*xp+b1;
		}else{
			xp=(b2-b1)/(a1-a2);
			yp=a1*xp+b1;
		}
		r=sqrt((xp-x1)*(xp-x1)+(yp-y1)*(yp-y1));

		printf("%.3f %.3f %.3f\n",xp,yp,r);
	}

	return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<queue>
#include<map>
#include<math.h>
#include<iomanip>
#include<set>
#include<numeric>
#include<cstring>
#include<cstdio>
#include<functional>
#include<bitset>
#include<limits.h>
#include<cassert>
#include<iterator>
#include<complex>
#include<stack>
#include<sstream>
#include<iterator>
using namespace std;
typedef  long long int lint;
typedef pair<int, int> IP;
typedef pair<lint, lint> LLP;
typedef pair<char, char>CP;
#define rep(i, n) for (int i = 0; i < n; i++)
#define repr(i, n) for (int i = n; i >= 0; i--)
#define sort(v) sort((v).begin(), (v).end())
#define reverse(v) reverse((v).begin(), (v).end())
#define upper(v,hoge) upper_bound(v.begin(),v.end(),hoge)
#define lower(v,hoge) lower_bound(v.begin(),v.end(),hoge)
#define llower(v,hoge) *lower_bound(v.begin(), v.end(), hoge)
#define lupper(v,hoge) *upper_bound(v.begin(), v.end(), hoge)

double a(double x1, double y1, double x2, double y2) {
	double a = 0;
	if (y1 == y2) {
		a = 114514;
	}
	else {
		a = -(x1 - x2) / (y1 - y2);
	}
	return a;
}
double b(double x1, double y1, double x2, double y2) {
	double b = 0;
	if (y1 == y2) {
		b = 114514;
	}
	else {
		b = ((x1 - x2) * (x1 + x2) / (y1 - y2) + (y1 + y2)) / 2;
	}
	return b;
}

double cal_X(double a, double b, double c, double d, double e, double f) {
	double Y = (c * d - a * f) / (b * d - a * e);
	double X;
	if (a != 0) {
		X = (c - b * Y) / a;
	}
	else {
		X = (f - e * Y) / d;
	}
	return X;
}

double cal_Y(double a, double b, double c, double d, double e, double f) {
	double Y = (c * d - a * f) / (b * d - a * e);
	double X;
	if (a != 0) {
		X = (c - b * Y) / a;
	}
	else {
		X = (f - e * Y) / d;
	}
	return Y;
}

int main() {
	int N;
	cin >> N;
	rep(i, N) {
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double a1 = a(x3, y3, x2, y2);
		double b1 = b(x3, y3, x2, y2);
		double a2 = a(x1, y1, x2, y2);
		double b2 = b(x1, y1, x2, y2);
		double a3 = a(x1, y1, x3, y3);
		double b3 = b(x1, y1, x3, y3);
		vector<double>V;
		if (a1 != 114514) {
			V.push_back(a1);
		}
		if (b1 != 114514) {
			V.push_back(b1);
		}
		if (a2 != 114514) {
			V.push_back(a2);
		}
		if (b2 != 114514) {
			V.push_back(b2);
		}
		if (a3 != 114514) {
			V.push_back(a3);
		}
		if (b3 != 114514) {
			V.push_back(b3);
		}
		double X = cal_X(V[0] * (-1), 1, V[1], V[2] * (-1), 1, V[3]);
		double Y = cal_Y(V[0] * (-1), 1, V[1], V[2] * (-1), 1, V[3]);
		double R = sqrt(pow(X - x1, 2) + pow(Y - y1, 2));
		cout <<fixed<<setprecision(3)<< X << ' ' << Y <<' '<< R << endl;
	}
}

C++: #include<stdio.h>
#include<math.h>

int main()
{
	int n;
	double x1[32],x2[32],x3[32],y1[32],y2[32],y3[32],p,q,r;
	double a,b,c;
	scanf("%d",&n);
	for(int i=0;i<n;i++){
		scanf("%lf%lf%lf%lf%lf%lf",&x1[i],&x2[i],&x3[i],&y1[i],&y2[i],&y3[i]);
		p=((y1[i]-y3[i])*(y1[i]*y1[i]-y2[i]*y2[i]+x1[i]*x1[i]-x2[i]*x2[i])-(y1[i]-y2[i])*(y1[i]*y1[i]-y3[i]*y3[i]+x1[i]*x1[i]-x3[i]*x3[i]));
		p/=(2*(y1[i]-y3[i])*(x1[i]-x2[i])-2*(y1[i]-y2[i])*(x1[i]-x3[i]));
		q=((x1[i]-x3[i])*(x1[i]*x1[i]-x2[i]*x2[i]+y1[i]*y1[i]-y2[i]*y2[i])-(x1[i]-x2[i])*(x1[i]*x1[i]-x3[i]*x3[i]+y1[i]*y1[i]-y3[i]*y3[i]));
		q/=(2*(x1[i]-x3[i])*(y1[i]-y2[i])-2*(x1[i]-x2[i])*(y1[i]-y3[i]));
		c=sqrt(fabs((x3[i]-x1[i])*(x3[i]-x1[i])+(y3[i]-y1[i])*(y3[i]-y1[i])));
		b=sqrt(fabs((x3[i]-x2[i])*(x3[i]-x2[i])+(y3[i]-y2[i])*(y3[i]-y2[i])));
		a=sqrt(fabs((x2[i]-x1[i])*(x2[i]-x1[i])+(y2[i]-y1[i])*(y2[i]-y1[i])));
		r=a*b*c/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
		printf("%.3f %.3f %.3f\n",p,q,r);
	}
	return 0;
}