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>
using namespace std;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.setf(ios::fixed);
    cout.precision(3);
    int n;
    cin>>n;
    for(int i=0; i<n; i++){
        double x1, y1, x2, y2, x3, y3;
        cin>>x1>>y1>>x2>>y2>>x3>>y3;
        double m=(y1*(x2-x3)+y2*(x3-x1)+y3*(x1-x2))*2;
        double cx=(x1*x1+y1*y1)*(y2-y3) +(x2*x2+y2*y2)*(y3-y1) +(x3*x3+y3*y3)*(y1-y2);
        double cy=(x1*x1+y1*y1)*(x2-x3) +(x2*x2+y2*y2)*(x3-x1) +(x3*x3+y3*y3)*(x1-x2);
        double x=-cx/m;
        double y=cy/m;
        double r=sqrt((x1-x)*(x1-x)+(y1-y)*(y1-y));
        cout<<x<<' '<<y<<' '<<r<<'\n';
    }
    return 0;
}

C++: #include <iostream>
#include <cmath>
#include <iomanip>

using namespace std;

    // 半径取得の処理
double getR(double x1, double y1, double x2, double y2, double x3, double y3) {
    
    double marginX1, marginX2, marginX3;
    double marginY1, marginY2, marginY3;
    double a, b, c;
    double r, s;
    
        // それぞれのxとyの長さを求める
    marginX1 = abs(x2 - x1);
    marginY1 = abs(y2 - y1);
    marginX2 = abs(x1 - x3);
    marginY2 = abs(y1 - y3);
    marginX3 = abs(x3 - x2);
    marginY3 = abs(y3 - y2);
    
        // それぞれの辺の長さを求める
    a = sqrt(pow(marginX1,2) + pow(marginY1,2));
    b = sqrt(pow(marginX2,2) + pow(marginY2,2));
    c = sqrt(pow(marginX3,2) + pow(marginY3,2));
    
        // 半径を求める
    s = (a + b + c) / 2;
    r = (a * b * c) / (4 * sqrt(s * (s - a) * (s - b) * (s - c)));
    
    r *= 1000;
    r = round(r);
    r /= 1000;
    
    return r;
}

    // 中心座標を求める処理
void getCentralCoodinate(double x1, double y1, double x2, double y2, double x3, double y3, double *xp, double *yp) {
    
    double marginX1, marginX2;
    double marginY1, marginY2;
    double marginXp1, marginXp2;
    double marginYp1, marginYp2;
    double n, m;
    
        // 初期化
    *xp = 0;
    *yp = 0;
    
        // それぞれのxとyの長さを求める
    marginX1 = abs(x2 - x1);
    marginY1 = abs(y2 - y1);
    marginX2 = abs(x1 - x3);
    marginY2 = abs(y1 - y3);
    
        // 中心座標を求めるのに必要な式
    marginXp1 = 2 * marginX1;
    marginYp1 = 2 * marginY1;
    marginXp2 = 2 * marginX2;
    marginYp2 = 2 * marginY2;
    
    n = pow(x1,2) - pow(x2,2) + pow(y1,2) - pow(y2,2);
    m = pow(x1,2) - pow(x3,2) + pow(y1,2) - pow(y3,2);
    
    *xp = (marginYp1 * m - marginYp2 * n) / (marginXp1 * marginYp2 - marginXp2 * marginYp1);
    *yp = (marginXp2 * n - marginXp1 * m) / (marginXp1 * marginYp2 - marginXp2 * marginYp1);
    
    *xp *= 1000;
    *xp = round(*xp);
    *xp /= 1000;
    
    *yp *= 1000;
    *yp = round(*yp);
    *yp /= 1000;
}

    // main処理
int main(void) {
    double x1, x2, x3;
    double y1, y2, y3;
    double xp, yp;
    double r;
    int n;
    
    cin >> n;
    
    while(cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
        
            // 半径と中心座標を取得する処理
        r = getR(x1, y1, x2, y2, x3, y3);
        getCentralCoodinate(x1, y1, x2, y2, x3, y3, &xp, &yp);
        
            // 出力
        cout << showpoint << setprecision(4);
        cout << xp << " " << yp << " " << r << endl;
    }
    
    return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>

double a,b,c,d,e,f,x,y,r;
int n;

double ifof(double a){
	int n = a*10000;
	if(n%10 >= 5)
		n++;
	return n/10000.0;
}

double solvex(){
	double k,l;
	k = (c*c)-(a*a)+(d*d)-(b*b)-(2*y*(d-b));
	l = 2*(c-a);
	return k/l;
}

double solvey(){
	double k,l;
	k = ((e-a)*((c*c)-(a*a)+(d*d)-(b*b)))-((c-a)*((e*e)-(a*a)+(f*f)-(b*b)));
	l = 2*(((d-b)*(e-a))-((f-b)*(c-a)));
	return k/l;
}

int main(){
	std::cin >> n;
	for(int i=0; i<n; i++){
		std::cin >> a >> b >> c >> d >> e >> f;
		y = ifof(solvey());
		x = ifof(solvex());
		r = ifof(std::sqrt(((x-a)*(x-a)) + ((y-b)*(y-b))));
		std::printf("%.3f %.3f %.3f\n",x,y,r);
	}
	return 0;
}

C++: #include<complex>
#include<stdio.h>
#define F(a,b,c) acos((b*b+c*c-a*a)/2/b/c)
int main(){typedef double D;int n;D d,e,f,g,h,i;for(scanf("%d",&n);n--;){scanf("%lf%lf%lf%lf%lf%lf",&d,&g,&e,&h,&f,&i);std::complex<D> a(d,g),b(e,h),c(f,i);float A=abs(b-c),B=abs(c-a),C=abs(a-b),L=F(A,B,C),s=sin(2*L),t=sin(2*F(B,C,A)),u=sin(2*F(C,A,B)),v=s+t+u;printf("%.3f %.3f %.3f\n",(d*s+e*t+f*u)/v,(g*s+h*t+i*u)/v,A/sin(L)/2);}}