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 <cstdio>
#include <complex>
#include <vector>
#include <cmath>

using namespace std;

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)

typedef complex<double> P;
typedef vector<P> VP;
const double EPF = 1e-5;

#define X real()
#define Y imag()
#define EQ(x,y) (abs(x-y) < EPF)

double dot(P a, P b) {
  return (conj(a)*b).X;
}
double cross(P a, P b) {
  return (conj(a)*b).Y;
}
double distPP(P a, P b) {
  return sqrt(dot(a-b,a-b));
}
P crosspoint(P a, P b, P c, P d) {//ac & bd
  double d1 = cross(d-b,b-a);
  double d2 = cross(d-b,c-a);
  if(EQ(d1,0) & EQ(d2,0)) return a;
  if(EQ(d2,0)) return a;
  return a + (c-a)*d1/d2;
}
P normal_vector(P a) {
  P b(a.Y, a.X);
  return b;
}

double t_r(double a) {
  return round(a * 1000)*1.0/1000;
}

int main() {
  int n;
  scanf("%d", &n);
  REP(j, n) {
    VP a;
    REP(i, 3) {
      double x,y;
      scanf("%lf %lf", &x, &y);
      a.push_back(P(x,y));
    }
    P M = (a[0]+a[1])/(P)2;
    P N = (a[1]+a[2])/(P)2;
    P M_v = normal_vector(a[0]-a[1]);
    P N_v = normal_vector(a[1]-a[2]);  
    P o = crosspoint(M, N, M+M_v, N+N_v);
    
    printf("%.3lf %.3lf %.3lf\n", t_r(o.X), t_r(o.Y), t_r(distPP(o,a[0])));
    a.clear();
  }
  return 0;
}

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

struct TCoordinate
{
	double x;
	double y;
};

struct TCircle
{
	double CenterX;
	double CenterY;
	double Radius;
};

TCircle GetCircle(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	
}

TCoordinate GetCenterPoint(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	double x1 = P1.x;
	double x2 = P2.x;
	double x3 = P3.x;
	double y1 = P1.y;
	double y2 = P2.y;
	double y3 = P3.y;
	double x, y, r;
	if(y1 == y2 || y2 == y3 || y3 == y1){
		if(y2 == y3){
			swap(x3, x1);
			swap(y3, y1);
		}else if(y3 == y1){
			swap(x2, x3);
			swap(y2, y3);
		}
		double xa = (x2 + x3) / 2;
		double ya = (y2 + y3) / 2;
		double a  = (-x3 + x2) / (y3 - y2);
		x = (x1 + x2) / 2;
		y = a * (x - xa) + ya;
	}else{
		double xa = (x1 + x2) / 2;
		double ya = (y1 + y2) / 2;
		double a  = (-x2 + x1) / (y2 - y1);
		double xb = (x2 + x3) / 2;
		double yb = (y2 + y3) / 2;
		double b  = (-x3 + x2) / (y3 - y2);
		x = (b * xb - a * xa - yb + ya) / (b - a);
		y = a * (x - xa) + ya;
	}
	return {x, y};
}

double GetRadius(TCoordinate P1, TCoordinate P2, TCoordinate P3)
{
	double x1 = P1.x;
	double x2 = P2.x;
	double x3 = P3.x;
	double y1 = P1.y;
	double y2 = P2.y;
	double y3 = P3.y;
	double x, y, r;
	if(y1 == y2 || y2 == y3 || y3 == y1){
		if(y2 == y3){
			swap(x3, x1);
			swap(y3, y1);
		}else if(y3 == y1){
			swap(x2, x3);
			swap(y2, y3);
		}
		double xa = (x2 + x3) / 2;
		double ya = (y2 + y3) / 2;
		double a  = (-x3 + x2) / (y3 - y2);
		x = (x1 + x2) / 2;
		y = a * (x - xa) + ya;
	}else{
		double xa = (x1 + x2) / 2;
		double ya = (y1 + y2) / 2;
		double a  = (-x2 + x1) / (y2 - y1);
		double xb = (x2 + x3) / 2;
		double yb = (y2 + y3) / 2;
		double b  = (-x3 + x2) / (y3 - y2);
		x = (b * xb - a * xa - yb + ya) / (b - a);
		y = a * (x - xa) + ya;
	}
	return sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1));
}

int main()
{
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		TCoordinate P1, P2, P3;
		cin >> P1.x >> P1.y >> P2.x >> P2.y >> P3.x >> P3.y;
		TCoordinate CenterPoint = GetCenterPoint(P1, P2, P3);
		double Radius = GetRadius(P1, P2, P3);
		cout << fixed << setprecision(3);
		cout << CenterPoint.x << ' ' << CenterPoint.y << ' ' << Radius << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    double a[2],b[2],c[2],o[2],m[2][2],x[2],y[2],det,i[2][2],r;
    cin >>a[0]>>a[1]>>b[0]>>b[1]>>c[0]>>c[1];
    b[0]-=a[0];
    b[1]-=a[1];
    c[0]-=a[0];
    c[1]-=a[1];
    m[0][0] = 2*b[0];
    m[0][1] = 2*b[1];
    m[1][0] = 2*c[0];
    m[1][1] = 2*c[1];
    y[0] = b[0]*b[0]+b[1]*b[1];
    y[1] = c[0]*c[0]+c[1]*c[1];
    det = m[0][0]*m[1][1]-m[1][0]*m[0][1];
    i[0][0] = m[1][1]/det;
    i[0][1] = -m[0][1]/det;
    i[1][0] = -m[1][0]/det;
    i[1][1] = m[0][0]/det;
    x[0] = y[0]*i[0][0]+y[1]*i[0][1];
    x[1] = y[0]*i[1][0]+y[1]*i[1][1];
    r = sqrt(x[0]*x[0]+x[1]*x[1]);
    x[0]+=a[0];
    x[1]+=a[1];
    printf("%.3lf %.3lf %.3lf\n",x[0],x[1],r);
  }
  return 0;
}

C++: #include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<cassert>
#include<iostream>
#include<sstream>
#include<string>
#include<vector>
#include<queue>
#include<set>
#include<map>
#include<utility>
#include<numeric>
#include<algorithm>
#include<bitset>
#include<complex>

using namespace std;

typedef long long Int;
typedef vector<int> vint;
typedef pair<int,int> pint;
#define mp make_pair

template<class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cout << *i << " "; cout << endl; }
template<class T> void chmin(T &t, T f) { if (t > f) t = f; }
template<class T> void chmax(T &t, T f) { if (t < f) t = f; }
int in() { int x; scanf("%d", &x); return x; }

struct Pt{
	double x,y;
	Pt(){}
	Pt(double x,double y) : x(x),y(y){}
	Pt operator+(const Pt &a) const {return Pt(x+a.x,y+a.y);}
	Pt operator-(const Pt &a) const {return Pt(x-a.x,y-a.y);}
	Pt operator*(const Pt &a) const {return Pt(x*a.x-y*a.y,x*a.y+y*a.x);}
	Pt operator*(const double &k) const {return Pt(x*k,y*k);}
	Pt operator/(const double &k) const {return Pt(x/k,y/k);}
	double abs() const {return sqrt(x*x+y*y);}
	double dot(const Pt &a) const {return x*a.x+y*a.y;}
	double det(const Pt &a) const {return x*a.y-y*a.x;}
};

int main() {
	Pt a,b,c;
	int n;
	n=in();
	int i;
	for(i=0;i<n;i++){
		cin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
		Pt bc=c-b,ca=a-c,ab=b-a;
		Pt res=(b+c-bc*Pt(0,1)*ca.dot(ab)/ca.det(ab))/2;
		printf("%.3f %.3f %.3f\n",res.x,res.y,(res-a).abs());
	}
	return 0;
}