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++: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace contest10
{
    class Program
    {
        static void Main(string[] args)
        {
            var read_line_origin = Console.ReadLine();

            if (string.IsNullOrEmpty(read_line_origin))
            {
                return;
            }

            //三角形の数
            int all_num = int.Parse(read_line_origin);

            
            //空白で分割する
            read_line_origin = Console.ReadLine();
            string[] words = read_line_origin.Split();

            for (int i = 0; i < all_num; i++)
            {
                //配列の要素、すべてに処理を行う
                var num_array = Array.ConvertAll(words, len => double.Parse(len));

                //三角形の座標
                double x1 = num_array[(i * 6) + 0];
                double y1 = num_array[(i * 6) + 1];
                double x2 = num_array[(i * 6) + 2];
                double y2 = num_array[(i * 6) + 3];
                double x3 = num_array[(i * 6) + 4];
                double y3 = num_array[(i * 6) + 5];

                //辺の長さ
                double r1 = Math.Sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
                double r2 = Math.Sqrt((x2 - x3) * (x2 - x3) + (y2 - y3) * (y2 - y3));
                double r3 = Math.Sqrt((x3 - x1) * (x3 - x1) + (y3 - y1) * (y3 - y1));

                //計算用
                double tmp_1 = (r1 * r1) * (r2 * r2 + r3 * r3 - r1 * r1);
                double tmp_2 = (r2 * r2) * (r3 * r3 + r1 * r1 - r2 * r2);
                double tmp_3 = (r3 * r3) * (r1 * r1 + r2 * r2 - r3 * r3);

                double tri_tmp = tmp_1 + tmp_2 + tmp_3;

                double center_x = (tmp_1 * x1 + tmp_2 * x2 + tmp_3 * x3) / tri_tmp;
                double center_y = (tmp_1 * y1 + tmp_2 * y2 + tmp_3 * y3) / tri_tmp;

                double x = (((x1 * x1 + y1 * y1) * (y2 - y3)) +
                            ((x2 * x2 + y2 * y2) * (y3 - y1)) +
                            ((x3 * x3 + y3 * y3) * (y1 - y2)))
                            /
                            ((x1 * (y2 - y3)) +
                             (x2 * (y3 - y1)) +
                             (x3 * (y1 - y2))
                            ) * 0.5;
                double y = (((x1 * x1 + y1 * y1) * (x2 - x3)) + ((x2 * x2 + y2 * y2) * (x3 - x1)) + ((x3 * x3 + y3 * y3) * (x1 - x2))) / ((y1 * (x2 - x3)) + (y2 * (x3 - x1)) + (y3 * (x1 - x2))) * 0.5;


                double radius = (r1 * r2 * r3) /
                                Math.Sqrt(
                                    (r1 + r2 + r3) * (-r1 + r2 + r3) * (r1 - r2 + r3) * (r1 + r2 - r3)

                                );

                Console.WriteLine("{0:f3} {1:f3} {2:f3}", x, y, radius);
            }




        }
    }
}

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

int main()
{
    int n;
    double x1,x2,x3,y1,y2,y3,A1,B1,C1,A2,B2,C2,X,Y;
    cin>>n;
    while(n!=0){
        scanf("%lf%lf%lf%lf%lf%lf",&x1,&x2,&x3,&y1,&y2,&y3);
        A1=2*(x2-x1);
        B1=2*(y2-y1);
        C1=x1*x1-x2*x2+y1*y1-y2*y2;
        A2=2*(x3-x1);
        B2=2*(y3-y1);
        C2=x1*x1-x3*x3+y1*y1-y3*y3;
        X=(B1*C2-B2*C1)/(A1*B2-A2*B1);
        Y=(C1*A2-C2*A1)/(A1*B2-A2*B1);
        printf("%.3lf %.3lf %.3lf\n",X,Y,sqrt((X-x1)*(X-x1)+(Y-y1)*(Y-y1)));
        n--;
    }
    return 0;
}