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 <iomanip>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <sstream>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <numeric>
#include <cmath>
#include <climits>
#include <limits>
#include <cfloat>
#include <fstream>
using namespace std;

int main()
{
//	cut here before submit 
//	freopen("testcase.cct", "r", stdin);
	string str = "";
	string res = "";
	int n;

	getline (cin, str );
	stringstream ssn(str);
	ssn >> n;

	int i,j;
	for (i = 0; i < n; ++i){
		getline (cin, str );
		stringstream ss(str);
		vector <double> X(3, 0.);
		vector <double> Y(3, 0.);
		for (j = 0; j < 3; ++j){
			double x, y;
			ss >> x >> y;
//			cout << '(' << x << ',' << y << ')' << endl;
			X[j] = x; Y[j] = y;
		} // end for
		vector <double> mx(3, 0.);
		vector <double> my(3, 0.);

		for (j = 0; j < 3; ++j){
			mx[j] = (X[j] + X[(j+1) % 3])/2.;
			my[j] = (Y[j] + Y[(j+1) % 3])/2.;
//			cout << '(' << mx[j]<< ',' << my[j] << ')' << endl;
		} // end for
		vector <double> a(3, 0.);
		vector <double> b(3, 0.);
		vector <double> c(3, 0.);

		for (j = 0; j < 3; ++j){
			if (X[(j+1) % 3] == X[j]){
				a[j] = 0.0;
				b[j] = 1.0;
				c[j] = -my[j];
			}else if (Y[(j+1) % 3] == Y[j]){
				a[j] = 1.0;
				b[j] = 0.0;
				c[j] = -mx[j];
			}else{
				a[j] = (X[(j+1) % 3] - X[j]);
				b[j] = (Y[(j+1) % 3] - Y[j]);
 				c[j] = -a[j]*mx[j] - b[j]*my[j];
			} // end if
		} // end for
		vector <double> cx (3, X[0]);
		vector <double> cy (3, Y[0]);
		set <double> Cx, Cy;
		for (j = 0; j < 3; ++j){
			//cout << a[j] << ' ' << b[j] << ' ' << c[j] << endl;
			if (a[j] == 0. && b[j] != 0.) {
				cy[j] = -c[j]/b[j];
			}else if (a[(j+1) % 3] == 0. && b[(j+1) % 3] != 0.){
				cy[j] = -c[(j+1) % 3]/b[(j+1) % 3];
			} // end if
			if (a[j] != 0. && b[j] == 0.){
				cx[j] = -c[j]/a[j];
			}else if (a[(j+1) % 3] != 0. && b[(j+1) % 3] == 0.){
				cx[j] = -c[(j+1) % 3]/a[(j+1) % 3];
			} // end if
			if (cx[j] == X[0] || cy[j] == Y[0] ){
				double D = a[j]*b[(j+1) % 3] - b[j]*a[(j+1) % 3];
				if (cx[j] == X[0]){
					cx[j] = ( -b[(j+1) % 3]*c[j] + b[j]*c[(j+1) % 3])/D;
				} // end if
				if (cy[j] == Y[0]){
					cy[j] = (  a[(j+1) % 3]*c[j] - a[j]*c[(j+1) % 3])/D;
				} // end if
			} // end if
			Cx.insert (cx[j]);
			Cy.insert (cy[j]);
	//		cout << '(' << cx[j] << ',' << cy[j] << ')' << endl;	
		} // end for
		double CX = *(Cx.begin() );
		double CY = *(Cy.begin() );
		double R = sqrt( (CX - X[0])*(CX - X[0]) + (CY - Y[0])*(CY - Y[0]) );

		cout.setf( ios::fixed ); 
		cout.precision(3);
		cout <<  CX << ' ' << CY << ' ' << R << endl;
 	} // end for

		
	return 0;
}

C++: #include<complex>
#include<cstdio>
#include<cmath>
using namespace std;
typedef complex<double> P;
#define F(a,b,c) acos((b*b+c*c-a*a)/2/b/c)
int main(){
	double n,d,e,f,g,h,i;
	for(scanf("%d",&n);n--;){
		scanf("%lf%lf%lf%lf%lf%lf",&d,&g,&e,&h,&f,&i);
		P a(d,g),b(e,h),c(f,i);
		double A=abs(b-c),B=abs(c-a),C=abs(a-b),
		s=sin(2*F(A,B,C)),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(F(A,B,C))/2);
	}
}

C++: #include<iostream>
#include<math.h>
#include<cstdio>
using namespace std;

int main()
{
    int n;
    int x1,x2,x3,y1,y2,y3;
    cin>>n;
    while(n--)
    {
        double x,y,r,dx,dy;
        cin>>x1>>y1>>x2>>y2>>x3>>y3;
        dx=(x2+x3-2*x1)/2;
        dy=(y2+y3-2*y1)/2;
        x=x1+dx;
        y=y1+dy;
        r=sqrt(dx*dx+dy*dy);
        printf("%.3f %.3f %.3f\n",x,y,r);
    }
    return 0;
}