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{
      if(p1.y == p2.y){
        ans.x = p12.x;
        ans.y = (-1/slp23) * ans.x + t2;
      }
      else if(p2.y == p3.y){
        ans.x = p23.x;
        ans.y = (-1/slp12) * ans.x + t1;
      }
      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 <cmath>

int main()
{
        int n;
        double x1, x2, x3, y1, y2, y3, a, b, c, r;
        std::cin >> n;
        std::cout.precision(3);
        std::cout.setf(std::ios::fixed);
        while(n--)
        {
                std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
                a = (x1 - x2)*(x1 - x2) + (y1 - y2)*(y1 - y2);
                b = (x1 - x3)*(x1 - x3) + (y1 - y3)*(y1 - y3);
                c = (x2 - x3)*(x2 - x3) + (y2 - y3)*(y2 - y3);
                r = -(a*a + b*b + c*c - 2*a*b - 2*b*c - 2*c*a);
                std::cout << (a*(-a+b+c)*x1 + b*(a-b+c)*x2 + c*(a+b-c)*x3)/r << " " << (a*(-a+b+c)*y1 + b*(a-b+c)*y2 + c*(a+b-c)*y3)/r << " " << sqrt(a*b*c/r) << std::endl;
        }
        return 0;
}

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

float a, b, c, d, e, f;
float x, y;

void solve(){
  float tmp1 = f-(c*d/a);
  float tmp2 = e-(b*d/a);
  y = tmp1/tmp2;
  x = (c - b * y) / a;
  printf("%.3f %.3f ", x, y);        
  //printf("y=%f\n", (f-(c*d/a))/(e-(b*d/a)));
  // ツ四ツ偲個古慊禿シ
  int tx = (int)(10000 * x);
  if (tx % 10 >= 5) {
    tx += 10;
  }
  tx /= 10;
  x = (float)tx / 1000.0f;

  int ty = (int)(10000 * y);
  if (ty % 10 >= 5) {
    ty += 10;
  }
  ty /= 10;
  y = (float)ty / 1000.0f;
}

int main(int argc, char const* argv[]) {
  int n;
  cin>>n;
  for (int i = 0; i < n; i += 1) {
    float x1, y1, x2, y2, x3, y3;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    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;
    solve();
    
    float r = (x - x1)*(x - x1) + (y - y1)*(y - y1);
    int tr = (int)(10000 * r);
    if (tr % 10 >= 5) {
      tr += 10;
    }
    tr /= 10;
    r = (float)tr / 1000.0f;
    
    printf("%.3f\n", sqrt(r));
  }  
  return 0;
}