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<algorithm>
#include<string>
#include<vector>
#include<cmath>
#define int long long
#define mod 1000000007
#define for0(i, n) for(int i = 0; i < (n); i++)
#define for1(i, n) for(int i = 1; i <= (n);i++)
using namespace std;
int get() { int r; cin >> r; return r; }
double dis(double p, double q, double r, double s) { return sqrt((p - r) * (p - r) + (q - s) * (q - s)); }
signed main() {
	int n; cin >> n;
	while (n--) {
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double p = ((x3 - x2) * (x3 - x1) + (y3 - y2) * (y3 - y1)) / ((x3 - x2) * (y2 - y1) + (y3 - y2) * (x1 - x2)) / 2;
		double x4 = (x1 + x2) / 2 + p * (y2 - y1);
		double y4 = (y1 + y2) / 2 + p * (x1 - x2);
		double r = dis(x1, y1, x4, y4);
		printf("%.3f %.3f %.3f\n", x4, y4, r);
	}
}

C++: #include<iostream>
using namespace std;
int main(){
	int n;
	double x[3], y[3], a, b, c, d, e, f, l , m, n;
	cin >> n;
	while(n--){
????????????????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]));
		
		cout << fixed << setprecision(3) << l << " " << m << " " << n << endl;
	}
}

C++: #include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
#define all(c) (c).begin(),(c).end()
#define pb push_back
#define mp make_pair
#define ERASE(v,i) (v).erase(remove(all(v),i),(v).end())
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(it,c) for(typeof((c).begin()) it=(c).begin();it!=(c).end();++it)
#define debug(x) cerr<<#x<<" = "<<(x)<<endl;
#define LINE cerr<<"LINE: "<<__LINE__<<endl;
inline int toInt(string s){int v;istringstream i(s);i>>v;return v;}
template<class T> inline string toString(T x){ostringstream o;o<<x;return o.str();}
template<class T> void pv(T a,T b){for(T it=a;it!=b;++it)cerr<<*it<<" ";cerr<<endl;}
const int INF = 1000000000;
const double PI = acos(-1.0), EPS = 1e-10;

inline double SQ(double x) {
	return x * x;
}

inline double DIST(double x1, double y1, double x2, double y2) {
	return sqrt(SQ(x1 - x2) + SQ(y1 - y2));
}

int main() {
	int N; cin >> N;
	while(N--) {
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		double A = x1 - x2;
		double B = y1 - y2;
		double C = x1 - x3;
		double D = y1 - y3;
		
		double S = SQ(x1) - SQ(x2) + SQ(y1) - SQ(y2);
		double T = SQ(x1) - SQ(x3) + SQ(y1) - SQ(y3);
		
		double X = (D * S - B * T) / 8;
		double Y = (-1 * C * S + A * T) / 8;
		
		printf("%.3f %.3f %.3f\n", X, Y, DIST(x1, y1, X, Y));
	}
	return 0;
}

C++: #include <stdio.h>
#include <math.h>
 
int main()
{
    int i, n;
    double x1, x2, x3, y1, y2, y3;
    double a, b, c, d, e, f;
    double X1, Y1, r;
     
    scanf("%d", &n);
    for (i = 0; i < n; i++){
        scanf("%lf %lf %lf %lf %lf %lf",&x1, &y1, &x2, &y2, &x3, &y3);
        a = 2 * (x2 - x1);
        b = 2 * (y2 - y1);
        c = x1*x1 - x2*x2 + y1*y1 - y2*y2;
        d = 2 * (x3 - x1);
        e = 2 * (y3 - y1);
        f = x1*x1 - x3*x3 + y1*y1 - y3*y3;
         
        X1 = (b*f - e*c) / (a*e - d*b);
        Y1 = (c*d - f*a) / (a*d - d*b);
        r  = sqrt((X1-x1)*(X1-x1) + (Y1-y1)*(Y1-y1));
         
        // X1 *= 1000;
        // Y1 *= 1000;
        // r  *= 1000;
         
        // X1 = (int)(X1 > 0.0 ? X1+0.5 : X1-0.5);
        // Y1 = (int)(Y1 > 0.0 ? Y1+0.5 : Y1-0.5);
        // r  = (int)(r  > 0.0 ? r +0.5 : r -0.5);
         
        // X1 = (double)X1/1000;
        // Y1 = (double)Y1/1000;
        // r  = (double)r /1000;
         
        printf("%.3f %.3f %.3f\n", X1, Y1, r);
         
         
    }
     
    return 0;
}