Go: package main

import (
	"fmt"
	"math"
)

func main() {
	n := scanInt()
	for i := 0; i < n; i++ {
		cal()
	}
}

func scanInt() int {
	var n int
	fmt.Scanf("%d", &n)
	return n
}

func cal() {
	var x1, y1, x2, y2, x3, y3 float64
	fmt.Scanf("%f%f%f%f%f%f", &x1, &y1, &x2, &y2, &x3, &y3)
	a := 2 * (x1 - x2)
	b := 2 * (y1 - y2)
	c := 2 * (x1 - x3)
	d := 2 * (y1 - y3)
	e := x1*x1 - x2*x2 + y1*y1 - y2*y2
	f := x1*x1 - x3*x3 + y1*y1 - y3*y3

	px := (d*e - b*f) / (a*d - b*c)
	py := (a*f - c*e) / (a*d - b*c)

	r := math.Sqrt((x1-px)*(x1-px) + (y1-py)*(y1-py))
	fmt.Printf("%.3f %.3f %.3f\n", px, py, r)
}

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

using namespace std;

#define SQ(x) ((x) * (x))

double ix[4], iy[4], x[4], y[4], a[4], b[4], c[4];

void equation(double x1, double y1, double x2, double y2, double *equa, double *equb)
{
  if(x1 == x2){
    *equb = 0;
    return;
  }
  if(y1 == y2){
    *equa = 0;
    return;
  }

  *equa = (y1 - y2) / (x1 - x2);
  return;
}

void line(double cx, double cy, double *equa, double *equb, double *equc)
{
  //  printf("%lf %lf %lf %lf %lf\n", cx, cy, *equa, *equb, *equc);
  if(*equa == 0){
    *equa = 1;
    *equb = 0;
    *equc = cx;
    //    printf("x = %lf\n", *equc);
    return;
  }
  if(*equb == 0){
    *equa = 0;
    *equb = 1;
    *equc = cy;
    //    printf("y = %lf\n", *equc);
    return;
  }

  *equa *= -1;
  *equc = cy - (*equa) * cx;
  return;
}

void point(double a1, double b1, double c1, double a2, double b2, double c2, double *rx, double *ry)
{
  if(!a1 || !b1 || !a2 || !b2 ){
    if(!a1 && !b2){
      *rx = c2;
      *ry = c1;
      return;
    }
    if(!b1 && !a2){
      *rx = c1;
      *ry = c2;
      return;
    }

    if(!a1){
      //      puts("a");
      *ry = c1;
      *rx = ((*ry) - c2) / a2;
      return;
    }
    if(!a2){
      //      puts("a");
      *ry = c2;
      *rx = ((*ry) - c1) / a1;
      return;
    }
    /*    if(!a1 || !a2){
      puts("a");
      double aa = (a1 * b2) - (a2 * b1);
      double cc = (c1 * b2) - (c2 * b1);
      *rx = -(cc / aa);
      *ry = a1 * (*rx) + c1;
      return;
      }*/

    if(!b1){
      //      puts("b1");
      *ry = (a2 * c1 + c2) / b2;
      *rx = a1;
      return ;
    }
    if(!b2){
      //      puts("b2");
      *ry = (a1 * c2 + c1) / b1;
      *rx = a2;
      return;
    }
    /*    if(!b1 || !b2){
      puts("b");
      double bb = (b1 * a2) - (b2 * a1);
      double cc = (c1 * a2) - (c2 * a1);
      *ry = cc / bb;
      *rx = ((*ry) * b1 - c1) / a1;
      return;
      }*/

    fprintf(stderr, "error\n");
    return;
  }

  double bb = (b1 * a2) - (b2 * a1);
  double cc = (c1 * a2) - (c2 * a1);
  *ry = cc / bb;
  *rx = ((*ry) * b1 - c1) / a1;
  return;
}

int main()
{
  int n;
  scanf("%d", &n);
  while(n--){
    for(int i=0;i<3;i++)
      scanf("%lf%lf", &ix[i], &iy[i]);
    //    printf("%lf %lf %lf %lf %lf %lf\n", ix[0], ix[1], ix[2], iy[0], iy[1], iy[2]);    
    a[0] = a[1] = b[0] = b[1] = 1;
    c[0] = c[1] = 0;
    equation(ix[0], iy[0], ix[1], iy[1], &a[0], &b[0]);
    equation(ix[1], iy[1], ix[2], iy[2], &a[1], &b[1]);

    //    printf("傾き %lf %lf %lf %lf\n", a[0], b[0], a[1], b[1]);


    line((ix[0]+ix[1])/2, (iy[0]+iy[1])/2, &a[0], &b[0], &c[0]);
    line((ix[1]+ix[2])/2, (iy[1]+iy[2])/2, &a[1], &b[1], &c[1]);

    //    printf("line %lf %lf %lf %lf %lf %lf\n", a[0], b[0], c[0], a[1], b[1], c[1]);
    
    double resx, resy;
    point(a[0], b[0], c[0], a[1], b[1], c[1], &resx, &resy);

    printf("%.3lf %.3lf %.3lf\n", resx, resy, sqrt(SQ(resx-ix[0]) + SQ(resy-iy[0])));
  }
  return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>
using namespace std;
int main()
{
long n,i;
double x1,y1,x2,y2,x3,y3,Px,Py,r,a1,b1,c1,a2,b2,c2;
cin>>n;
for(i=0;i<n;i++){
cin>>x1>>y1>>x2>>y2>>x3>>y3;
a1=(x2-x1)*2;
b1=(y2-y1)*2;
c1=x1*x1-x2*x2+y1*y1-y2*y2;
a2=(x3-x1)*2;
b2=(y3-y1)*2;
c2=x1*x1-x3*x3+y1*y1-y3*y3;
Px=(b1*c2-b2*c1)/(a1*b2-a2*b1);
Py=(c1*a2-c2*a1)/(a1*b2-a2*b1);
r=sqrt((x1-x)*(x1-x)+(y1-y)*(y1-y));
printf("%.3f %.3f %.3f\n",Px,Py,r);
}
return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <cstdio>
#include <cmath>
#include <string>
#include <cctype>
#include <stack>
#include <queue>
#include <vector>
#include <cstdlib>
#include <list>
#define _USE_MATH_DEFINES
#include <math.h>

using namespace std;

int main() {

	int n;
	double x1, x2, x3, y1, y2, y3;
	double X, Y, R;
	double a, b, c, d, e, f;

	cin >> n;

	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		a = -x2 + x3;
		b = -x1*x1 + x2*x2 - y1*y1 + y2*y2;
		c = x1 - x2;
		d = -x2*x2 + x3*x3 - y2*y2 + y3*y3;
		e = -c * 2 * (y2 - y3);
		f = a * 2 * (y1 - y2);

		Y = (a*b + c*d) / (e - f);
		X = (b + 2 * Y*(y1 - y2)) / (-2 * c);
		R = sqrt((X - x1)*(X - x1) + (Y - y1)*(Y - y1));

		//?°???°????¬¬??????????????¨?????\
		if (X > 0)
			X = (int)(X * 1000 + 0.5) ;
		else X = (int)(X * 1000 - 0.5) ;

		if (Y>0)
			Y = (int)(Y * 1000 + 0.5) ;
		else Y = (int)(Y * 1000 - 0.5) ;

		if (R > 0)
			R = (int)(R * 1000 + 0.5);
		else R = (int)(R * 1000 - 0.5);

		printf("%.3f %.3f %.3f", X/1000, Y/1000, R/1000);
	}

	return 0;
}

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

using namespace std;

int main(){
  int n;
  cin>>n;
  for (int i=0;i<n;i++){
    double x1,x2,x3,y1,y2,y3;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    double a,b,c,d,e,f;
    a=-2*x1+2*x2;
    b=-2*y1+2*y2;
    c=x2*x2+y2*y2-x1*x1-y1*y1;
    d=-2*x1+2*x3;
    e=-2*y1+2*y3;
    f=x3*x3+y3*y3-x1*x1-y1*y1;
    double x=(c*e-b*f)/(a*e-b*d),y=(c*d-a*f)/(b*d-a*e);
    double r=(x1-x)*(x1-x)+(y1-y)*(y1-y);
    r=sqrt(r);
    printf("%.3f %.3f %.3f\n",x,y,r);
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <math.h>
#include <vector>
#include <set>

using namespace std;
typedef long long LL;
static const double EPS = 1e-9;

#define FOR(i,k,n) for (int i=(k); i<(int)(n); ++i)
#define REP(i,n) FOR(i,0,n) 
#define A (x[1] - x[0])
#define B (y[1] - y[0])
#define C ((x[1]*x[1] - x[0]*x[0] + y[1]*y[1] - y[0]*y[0])/2)
#define D (x[2] - x[1])
#define E (y[2] - y[1])
#define F ((x[2]*x[2] - x[1]*x[1] + y[2]*y[2] - y[1]*y[1])/2)

int main(void){  
  double x[4],y[4];
  int n;
  cin >> n;
  while(n--){
    REP(i,3)
      cin >> x[i] >> y[i] ;
    x[3] = (C*E - B*F)/(A*E - B*D);
    y[3] = (C*D - A*F)/(B*D - A*E);
    printf("%.3f %.3f %.3f\n",x[3],y[3],sqrt((x[3]-x[0])*(x[3]-x[0]) + (y[3] -y[0])*(y[3]-y[0])));
  }
  return 0;
}

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

using namespace std;

long double sisya(long double a) {
	a *= 1000.0;
	//cout << a << endl;
	a += 0.5;
	a = (int)a;
	a /= 1000.0;
	return a; 
}

long double ll(long double x1, long double y1, long double x2, long double y2) {
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		long double x[3], y[3];
		for (int j = 0; j < 3; j++) {
			cin >> x[j] >> y[j];
		}
		long double a1, a2, b1, b2, c1, c2;
		a1 = 2.0 * (x[1]-x[0]);
		a2 = 2.0 * (x[2]-x[0]);
		b1 = 2.0 * (y[1]-y[0]);
		b2 = 2.0 * (y[2]-y[0]);
		c1 = x[0]*x[0]-x[1]*x[1]+y[0]*y[0]-y[1]*y[1];
		c2 = x[0]*x[0]-x[2]*x[2]+y[0]*y[0]-y[2]*y[2];
		long double X, Y, R;
		X = (b1*c2-b2*c1)/(a1*b2-a2*b1);
		Y = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		R = ll(X, Y, x[0], y[0]);
		X = sisya(X);
		Y = sisya(Y);
		R = sisya(R);
		printf("%.3f %.3f %.3f\n", X, Y, R);
	}
	return 0;
}