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 <cstdio>
#include <cstring>
#include <algorithm>
#include <complex>
using namespace std;

#define x real()
#define y imag()
typedef complex<double> Point;

Point circumscribed_Circle_Center(Point a, Point b, Point c){
  double A = a.x - b.x;
  double B = a.y - b.y;
  double C = -1 * (a.x*a.x - c.x*c.x + a.y*a.y - b.y*b.y);
  double D = a.x - c.x;
  double E = a.y - c.y;
  double F = -1 * (a.x*a.x - c.x*c.x + a.y*a.y - c.y*c.y);

  double L = (C*E - B*F) / (E*A - B*D);
  double M = (D*C - A*F) / (D*B - E*A);
  double xp = L / -2.0;
  double yp = M / -2.0;
  return (Point(xp, yp));
}

double circumscribed_Circle_Radius(Point a, Point c){
  double L = c.x * -2.0;
  double M = c.y * -2.0;
  double N = -1 * (a.x*a.x + a.y*a.y + L*a.x + M*a.y);
  double r = sqrt(L*L + M*M - 4*N) / 2.0;
  return r;
}

int main(){
  int n;
  Point a, b, c;
  cin >> n;
  while(n--){
    cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;
    Point C = circumscribed_Circle_Center(a, b, c);
    double r = circumscribed_Circle_Radius(a, C);
    printf("%.3f %.3f %.3f\n", C.x, C.y, r);
  }
  return 0;
}

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

using namespace std;
typedef complex<double> P;

double cross(P a,P b){return (conj(a)*b).imag();}

int main(){
  int n;
  scanf("%d",&n);
  while(n--){
    P a,b,c;
    cin >> a.real() >> a.imag() >> b.real() >> b.imag() >> c.real() >> c.imag();
    P m = (a+b)/P(2,0);
    P n = (a+c)/P(2,0);
    P s = (a-b)*P(0,1);
    P t = (a-c)*P(0,1);
    //    cout << " m: "<< m << " n: " << n << " s: " << s << " t: " <<  t << endl;
    double e = cross(n-m,t)/cross(s,t);
    //    cout << " e: " << e << endl;
    P o = m+s*P(e,0);
    double r = abs(a-o);
    printf("%.3f %.3f %.3f\n",o.real(),o.imag(),r);
  }
}

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

double fpx(double a, double b, double c, double d, double e, double f);
double fpy(double a, double b, double c, double d, double e, double f);
double  fr(double a, double b, double c, double d);
string toStr(double a);
string intToString(int a);

int main() {
  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 px = fpx(x1, y1, x2, y2, x3, y3);
    double py = fpy(x1, y1, x2, y2, x3, y3);
    double  r =  fr(x1, y1, px, py);

    cout << toStr(px) << " " << toStr(py) << " " << toStr(r) << endl;
  }
}


double fpx(double x1, double y1, double x2, double y2, double x3, double y3) {
  double a1, a2, b1, b2, c1, c2;

  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;

  return (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
}

double fpy(double x1, double y1, double x2, double y2, double x3, double y3) {
  double a1, a2, b1, b2, c1, c2;

  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;

  return (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
}

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

string toStr(double d) {
  string pm = "", val = "";
  if (d < 0) {
    pm = "-";
    d = -d;
  }

  int ipart = (int)d;
  int fpart = (int)( (d - (double)ipart) * 10000) + 10005;

  val = pm + intToString(ipart) + "." + intToString(fpart).substr(1, 3);
  return val;
}

string intToString(int n) {
  stringstream ss;
  ss << n;
  return ss.str();
}