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 <bits/stdc++.h>
using namespace std ;

#define pb(n) push_back(n)
#define fi first
#define se second
#define X real()
#define Y imag()
#define all(r) (r).begin(),(r).end()
#define gsort(st,en) sort((st),(en),greater<int>())
#define vmax(ary) *max_element(all(ary))
#define vmin(ary) *min_element(all(ary))
#define debug(x) cout<<#x<<": "<<x<<endl
#define fcout(n) cout<<fixed<<setprecision((n))
#define scout(n) cout<<setw(n)
#define vary(type,name,size,init) vector< type> name(size,init)

#define rep(i,n) for(int i = 0; i < (int)(n);++i)
#define REP(i,a,b) for(int i = (a);i < (int)(b);++i)
#define repi(it,array) for(auto it = array.begin(),end = array.end(); it != end;++it)
#define repa(n,array) for(auto &n :(array))

using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using dict = map<string,int>;
using comd = complex<double>;
using pii = pair<int,int> ;

constexpr int imax = ((1<<30)-1)*2+1 ;
constexpr int inf = 100000000;
constexpr double PI = acos(-1.0) ;
double eps = 1e-10 ;
const int dy[] = {-1,0,1,0};
const int dx[] = {0,-1,0,1};

inline bool value(int x,int y,int w,int h){
  return (x >= 0 && x < w && y >= 0 && y < h);
}

double CalcDist(comd p1, comd p2){
  return sqrt(pow(p1.X - p2.X,2.0) + pow(p1.Y -p2.Y,2.0));
}

template<typename T>
void Unique(vector<T> &v){
  sort(all(v));
  v.erase(all(v),v.end());
}

double EPS = 1e-10;
double add(double a, double b){
  if(abs(a+b) < EPS* (abs(a) + abs(b))) return 0;
  return a+b;
}

struct P{
  double x,y;
  P(){}
  P(double _x,double _y):x(_x),y(_y){}
  P operator + (P p){
    return P(add(x,p.x) ,add(y,p.y));
  }
  P operator - (P p){
    return P(add(x,-p.x) ,add(y,-p.y));
  }
  P operator * (double d){
    return P(x * d,y * d);
  }
  friend ostream& operator<<(ostream& os,const P& p){
    os << "("<<p.x <<","<<p.y<<")";
    return os;
  }
  double dot(P p){
    return add(x * p.x,y*p.y);
  }
  double det(P p){
    return add(x*p.y,-y*p.x);
  }
  double dist2(P p){
    return ((*this-p).x * (*this-p).x) + ((*this-p).y * (*this-p).y);
  }
  P GetCenter(P &p){
    return P((*this+p).x/2,(*this+p).y/2);
  }
  double GetSlope(P &p){
    P tmp = *this - p;
    return tmp.y/tmp.x;
  }
};
//???????????????????????¨?????????
bool on_seg(P p1,P p2,P q){
  return (p1-q).det(p2-q) == 0 && (p1-q).dot(p2-q) <= 0;
}
//???????°????
P intersection(P p1, P p2, P q1,P q2){
  return p1 + (p2-p1) * ((q2 - q1).det(q1-p1) / (q2 - q1).det(p2-p1)) ;
}
int is_prime[100001];
void Eratosthenes(int N){
  for(int i = 0; i < N; i++){
    is_prime[i] = 1;
  }
  is_prime[1] = 0;
  for(int i = 2; i*i < N ; i++){
    if(is_prime[i]){
      for(int j = 0; i * (j + 2) < N; j++){
        is_prime[i *(j + 2)] = 0;
      }
    }
  }
}
int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  P p1,p2,p3;
  cin >> n;
  rep(i,n){
    cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y;
    P p12,p23,ans;
    double slp12,slp23,t1,t2;
    p12 = p1.GetCenter(p2);
    p23 = p2.GetCenter(p3);
    slp12 = p1.GetSlope(p2),slp23 = p2.GetSlope(p3);
    if(p1.y == p2.y && p2.x == p3.x){
      ans.x = p12.x;
      ans.y = p23.y;
    }
    else if(p2.y == p3.y && p1.x == p2.x){
      ans.x = p23.x;
      ans.y = p12.y;
    }
    else{
      t1 = p12.y + p12.x / slp12;
      t2 = p23.y + p23.x / slp23;
      ans.x = (t1-t2)/(1/slp12 - 1/slp23);
      ans.y = (-1/slp12) * ans.x + t1;
    }
    fcout(3) << ans.x << " " << ans.y << " " << sqrt(ans.dist2(p1))<<endl;
  }
  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>0.001){
	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;
}

C++: #include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <map>
#include <algorithm>

#define rep(i, j) FOR(i, 0, j)
#define FOR(i, j, k) for(int i = j; i < k; ++i)
#define SQ(i) ((i) * (i))
using namespace std;

int main(){
  double x1, y1, x2, y2, x3, y3;
  double a1, b1, c1, a2, b2, c2, x, y, a, b, c, cos_a, sin_a, r;
  int n;
  scanf("%d", &n);
  while(n--){
    scanf("%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3);
    a1 = 2 * (x2 - x1); a2 = 2 * (x3 - x1);
    b1 = 2 * (y2 - y1); b2 = 2 * (y3 - y1);
    c1 = SQ(x1) - SQ(x2) + SQ(y1) - SQ(y2);
    c2 = SQ(x1) - SQ(x3) + SQ(y1) - SQ(y3);
    x = (b1 * c1 - b2 * c1) / (a1 * b2 - a2 * b1);
    y = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);

    r = sqrt((x-x1) + (y - y1));
    printf("%.3f %.3f %.3f\n",x, y, r);
  }
  return 0;
}

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

int main()
{
    int num;
    cin >> num;
    double x1, x2, x3, y1, y2, y3;
    double px, py, r;

    for(int i = 0; i < num; ++i)
    {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        px = (1 / 2.0) * ((x1 * x1 + y1 * y1) * (y3 - y2) + (x2 * x2 + y2 * y2) * (y1 - y3)
            + (x3 * x3 + y3 * y3) * (y2 - y1)) / (x1 * (y3 - y1) + x2 * (y1 - y3) + x3 * (y2 - y1));
        py = -(x3 - x2) / (y3 - y2) * px + (1 / 2.0) * (x3 * x3 - x2 * x2 + y3 * y3 - y2 * y2) / (y3 - y2);
        r = sqrt((x1 - px) * (x1 - px) + (y1 - py) * (y1 - py));

        printf("%.3f %.3f %.3f\n", px, py, r);
        //cout << fixed << setprecision(3) << round(px * 1000) / 1000 << " " << round(py * 1000) / 1000 << " " << round(r * 1000) / 1000 << endl;
    }
}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;
typedef pair<double, double> PDD;
PDD calc(double mat[2][2], double vec[2]){
  double a = mat[0][0], b = mat[0][1], d = mat[1][0], e = mat[1][1];
  double c = vec[0], f = vec[1];
  double det = a * e - d * b;
  return make_pair((e * c - b * f) / det, (-c * d + a * f) / det);
}
int main(){
  int Tc;
  cin >> Tc;
  for(int tc = 0; tc < Tc; ++tc){
    double x1, y1, x2, y2, x3, y3;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double A, B, C, D, E, F;
    A = (y1 + y3) / 2.;
    B = (x1 - x3) / (y1 - y3);
    C = (x1 + x3) / 2.;
    D = (y2 + y3) / 2.;
    E = (x2 - x3) / (y2 - y3);
    F = (x2 + x3) / 2.;
    double mat[2][2];
    mat[0][0] = B; mat[0][1] = 1;
    mat[1][0] = E; mat[1][1] = 1;
    double vec[2];
    vec[0] = A + B * C;
    vec[1] = D + E * F;
    PDD res = calc(mat, vec);
    double x = res.first, y = res.second;
    double r = pow((x-x1)*(x-x1)+(y-y1)*(y-y1), 0.5);
    x = (double)(round(x*1000))/1000.0;
    y = (double)(round(y*1000))/1000.0;
    r = (double)(round(r*1000))/1000.0;
    x = (x == -0.0) ? 0.0 : x;
    y = (y == -0.0) ? 0.0 : y;
    printf("%.3f %.3f %.3f\n", x, y, r);
  }
  return 0;
}