Go: package main

import (
	"fmt"
	"math"
)

func main() {
	var num int
	fmt.Scan(&num)
	for i := 0; i < num; i++ {
		var x1, y1, x2, y2, x3, y3 float64
		fmt.Scanf("%f%f%f%f%f%f", &x1, &y1, &x2, &y2, &x3, &y3)

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

		// 直線1: ax + by - e = 0
		// 直線2: cx + dy - f = 0
		px := (d*e - b*f) / (a*d - b*c)
		py := (c*e - a*f) / (b*c - a*d)
		r := math.Sqrt((px-x1)*(px-x1) + (py-y1)*(py-y1))
		fmt.Printf("%.3f %.3f %.3f\n", px, py, r)
	}
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<cstdio>
#include<climits>
#include<cmath>
#include<cstring>
#include<string>
#include<complex>

#define f first
#define s second
#define mp make_pair

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); i++)
#define ALL(c) (c).begin(), (c).end()
#define EPS (1e-10)
using namespace std;

typedef unsigned int uint;
typedef long long ll;

#define PI (3.1415926535)
#define EPS (1e-10)
bool eq(double a, double b){
  return abs(a-b) < EPS;
}

int main(){
  complex<double> p[3];
  int n; cin>>n;
  while(n --> 0){
    REP(i,3) cin>>p[i].real()>>p[i].imag();
    double ag = arg(p[0]-p[2])-arg(p[1]-p[2]);
    double r = abs(abs(p[0] - p[1]) / sin(ag) / 2.0);
    double s = r * r * sin(2*ag) / 2.0;
    complex<double> c = (p[0] + p[1]) / 2.0;
    complex<double> v = (p[0] - p[1]) * complex<double>(0,1);
    double rr = s * 2.0 / abs(p[0] - p[1]); 
    complex<double> o1 = c + rr * v / abs(v),
                    o2 = c - rr * v / abs(v), o;
    if(eq(abs(o1-p[0]),r) && eq(abs(o1-p[1]),r) && eq(abs(o1-p[2]),r)) o = o1;
    else o = o2;
    printf("%.3f %.3f %.3f\n",o.real(),o.imag(),r);
  }
  return 0;
}

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

using namespace std;

int main(void){
	int n;
	float x[3],y[3],a,b,c,d,X,Y,px,py;

	cin >> n;
	for(int i=0;i<n;i++){
		for(int j=0;j<3;j++)
			cin >> x[j] >> y[j];

		a=x[0]-x[1];
		b=y[0]-y[1];
		c=x[0]-x[2];
		d=y[0]-y[2];
		X=pow(x[0],2)-pow(x[1],2)+pow(y[0],2)-pow(y[1],2);
		Y=pow(x[0],2)-pow(x[2],2)+pow(y[0],2)-pow(y[2],2);
		px=(d*X-b*Y)/(a*d-b*c)/2;
		py=(-c*X+a*Y)/(a*d-b*c)/2;

		printf("%.3f %.3f %.3f\n",px,py,sqrt(pow(x[0]-px,2)+pow(y[0]-py,2)));
	}
	return 0;
}

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

/*
 * ax+by=c, dx+ey=f ツづーツ嘉ーツつュツ。
 * ツ姪淞づィツ値ツつェ0ツづ按づァツ青ャツ古キツ、1ツづ個湘ェツ債づ溝e-bdツつェEPSツ暗按可コツづ可づ按づづつ「ツづ古ォツ債キツ妥・ツつォツつ「ツ。
 */
int solve_linear_equations( double a, double b, double c, 
  double d, double e, double f, double& x, double& y )
{
  double EPS = 1e-9;
  double delta = a*e-b*d;
  x = 0.0;
  y = 0.0;
  if( (delta < 0 && -delta < EPS) || (delta >=0 && delta < EPS) ) return 1;
  x = ((c*e-b*f) / delta) + 0.0;
  y = ((a*f-c*d) / delta) + 0.0;
  return 0;
}

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

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

int main(){

  double x[3], y[3], mx, my, lx, ly, dm, dl, cx, cy, r;
  
  for(int i=0; i<3; i++){
    cin >> x[i] >> y[i];
  }
  
  mx = (x1 + x2) / 2;
  my = (y1 + y2) / 2;
  dm = (y1 - y2) / (x1 - y2);
  lx = (x2 + x3) / 2;
  ly = (y2 + y3) / 2;
  dl = (y2 - y3) / (x2 - x3);
  cx = (my + dm * mx) - (ly + dl * lx);
  cy = (ly + dl * lx) * dm - (my + dm * mx) * dl;
  
  r = sqrt((x1 - cx) * (x1 - cx) + (y1 - cy) * (y1 - cy));

  cx = round(cx * 1000) / 1000;
  cy = round(cy * 1000) / 1000;
  r = round(r * 1000) / 1000;

  printf("%.3d %.3d %.3d\n", cx, cy, r);

  return 0;
}

C++: #include<stdio.h>
#include<math.h>
int main(){
    int a;
    scanf("%d",&a);
    while(a--){
        double x1,y1,x2,y2,x3,y3;
        scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
        double a1=2*(x2-x1);
        double b1=2*(y2-y1);
        double c1=x1*x1-x2*x2+y1*y1-y2*y2;
        double a2=2*(x3-x1);
        double b2=2*(y3-y1);
        double c2=x1*x1-x3*x3+y1*y1-y3*y3;
        double x=(b1*c2-b2*c1)/(a1*b2-a2*b1);
        double y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
        double r=hypot(x-x1,y-y1);
        printf("%.3lf %.3lf %.3lf\n",x,y,r);
    }
}

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

const double EPS = 1E-8;
typedef vector<double> vd;
typedef vector<vd> mat;

// 拡大係数行列で解く
vector<double> GaussJordan(const mat& A, const vd b) {
  int n = A.size();
  mat B(n, vd(n+1));
  for(int i=0; i<n; i++) {
    for(int j=0; j<n; j++) B[i][j] = A[i][j];
  }
  for(int i=0; i<n; i++) {
    B[i][n] = b[i];
  }

  for(int i=0; i<n; i++) {
    int pivot = i;
    for(int j=i; j<n; j++) {
      if(abs(B[j][i]) > abs(B[pivot][i])) pivot = j;
    }
    swap(B[i], B[pivot]);

    if(abs(B[i][i]) < EPS) return vd();
    
    for(int j=i+1; j<=n; j++) B[i][j] /=B[i][i];
    for(int j=0; j<n; j++) {
      if(i != j) {
	for(int k=i+1; k<=n; k++) B[j][k] -= B[j][i] * B[i][k];
      }
    }
  }
  vd x(n);
  for(int i=0; i<n; i++) x[i] = B[i][n];
  return x;
}

int main() {
  int tcase;
  cin >> tcase;
  while(tcase--) {
    double x[3], y[3];
    cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
    
    mat A(3, vd(3, 0));
    for(int i=0; i<3; i++) {
      for(int j=0; j<3; j++) {
	if(j == 2) A[i][j] = 1;
	else A[i][j] = (j % 2 ? y[i] : x[i]);
      }
    }
    
    vd b(3);
    for(int i=0; i<3; i++) b[i] = -(x[i]*x[i] + y[i]*y[i]);

    vd v = GaussJordan(A, b);
    
    double cX = -v[0]/2, cY = -v[1]/2;
    double r = sqrt(pow(x[0]-cX, 2) + pow(y[0]-cY, 2));
    printf("%.3f %.3f %.3f\n", cX, cY, r);
  }
  
  return 0;
}

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

int main() {
	int n;
	cin>>n;
	for(int h=0;h<n;h++){
double x1,y1,x2,y2,x3,y3,x2m,x3m,y2m,y3m,x21,y21,x31,y31,xy21,xans,yans;
cin>>x1>>y1>>x2>>y2>>x3>>y3;
	x2m=(x1+x2)/2;
	y2m=(y1+y2)/2;
	x3m=(x1+x3)/2;
	y3m=(y1+y3)/2;
	x21=x2-x1;
	x31=x3-x1;
	y21=y2-y1;
	y31=y3-y1;
	
	
if(y1!=y2){
	xy21=x21/y21;
	xans=(x31*x3m-y31*(xy21*x2m+y3m-y2m))/(x31-y31*xy21);
	yans=-xy21*(xans-x2m)+y2m;
}	
else{
	xans=x2m;
	yans=(x3m-xans)*x31/y31+y3m;
}
double r;
r=sqrt((xans-x1)*(xans-x1)+(yans-y1)*(yans-y1));
double x=xans,y=yans;
	printf("%.3f %.3f %.3f\n",xans,yans,r);
}

	return 0;
}