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 FOR(i, s, n) for(int i = s; i < (int)n; i++)
#define per(i, n) for(int i = n; i >= 0; i--)
#define ROF(i, s, n) for(int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n) for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define CLR(a) memset((a), 0 ,sizeof(a))
#define NCLR(a) memset((a), -1 ,sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\n";
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << "\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue< int, vector<int>, greater<int> > minpq;
static const double EPS = 1e-10;
static const double PI = acos( -1.0 );
static const int mod = 1000000007;
static const int INF = 1 << 25;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = { -1, 0, 1, 0, 1, -1, 1, -1 };
static const int dy[] = { 0, -1, 0, 1, 1, 1, -1, -1 };

int n;
double x_1, y_1, x_2, y_2, x_3, y_3, x_4, y_4;

int main() {
	scanf( "%d", &n );
	for ( int i = 0; i < n; i++ ) {
		scanf( "%lf %lf %lf %lf %lf %lf", &x_1, &y_1, &x_2, &y_2, &x_3, &y_3 );
		double a1 = 2 * ( x_2 - x_1 );
		double b1 = 2 * ( y_2 - y_1 );
		double c1 = sq( x_1 ) - sq( x_2 ) + sq( y_1 ) - sq( y_2 );
		double a2 = 2 * ( x_3 - x_1 );
		double b2 = 2 * ( y_3 - y_1 );
		double c2 = sq( x_1 ) - sq( x_3 ) + sq( y_1 ) - sq( y_3 );
		double x = ( b1 * c2 - b2 * c1 ) / ( a1 * b2 - a2 * b1 );
		double y = ( c1 * a2 - c2 * a1 ) / ( a1 * b2 - a2 * b1 );

		double r = sqrt( sq( x - x_1 ) + sq( y - y_1 ) );
		printf( "%.3f %.3f %.3f\n", x, y, r );
	}
	return 0;
}

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

double a, b, c, d, e, f;
double x, y;
double rx, ry;


double RoundNumber(double f) {
  int tf = (int)(10000 * f);
  if (tf % 10 >= 5) {
    tf += 10;
  }
  tf /= 10;
  return (double)tf / 1000.0f;
}

void solve(){
  double tmp1 = f-(c*d/a);
  double tmp2 = e-(b*d/a);
  y = tmp1/tmp2;
  x = (c - b * y) / a;  
  printf("%.3f %.3f ", RoundNumber(x), RoundNumber(y));        
}

int main(int argc, char const* argv[]) {
  int n;
  cin>>n;
  for (int i = 0; i < n; i += 1) {
    double 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();
    
    double r = (x - x1)*(x - x1) + (y - y1)*(y - y1);
    r = sqrt(r);
    printf("%.3f\n", RoundNumber(r));
  }  
  return 0;
}

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

double EPS = 1e-10;
const double PI = acos(-1);

using namespace std;

double add(double a, double b){
  if(abs(a+b) < EPS * (abs(a)+abs(b)))return 0;
  return a+b;
}

struct point{
  double x, y;
  point(){}
  point(double x,double y) : x(x) , y(y){}

  point operator + (point p){
    return point(add(x,p.x), add(y,p.y));
  }

  point operator - (point p){
    return point(add(x,-p.x), add(y,-p.y));
  }

  point operator * (double d){
    return point(x*d,y*d);
  }

  point operator / (double d){
    return point(x/d,y/d);
  }
  
  bool operator == ( const point &p ) const {
    return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS;
  }
};

struct circle{
  point p;
  double r;
};

circle circumscribed_circle(point p, point q, point r){
  double a[2],b[2],c[2];
  circle res;
    
    a[0]=2*(q.x-p.x);
    b[0]=2*(q.y-p.y);
    c[0]=p.x*p.x-q.x*q.x+p.y*p.y-q.y*q.y;
    
    a[1]=2*(r.x-p.x);
    b[1]=2*(r.y-p.y);
    c[1]=p.x*p.x-r.x*r.x+p.y*p.y-r.y*r.y;
    
    res.p.x=(b[0]*c[1]-b[1]*c[0])/(a[0]*b[1]-a[1]*b[0]);
    res.p.y=(c[0]*a[1]-c[1]*a[0])/(a[0]*b[1]-a[1]*b[0]);
 
    res.r=sqrt(pow(fabs(p.x-res.p.x),2)+pow(fabs(p.y-res.p.y),2));
    
    return res;
}

int main(void) {
  
  int n;
  point in[3];
  circle ans;
 
  cin >> n;
  
  while(n--){
    for(int i=0;i<3;i++)
      cin >> in[i].x >> in[i].y;

    ans=circumscribed_circle(in[0], in[1], in[2]);

    printf("%.3f %.3f %.3f\n",ans.p.x,ans.p.y,ans.r);
  }
  
  return 0;
}

C++: // includes
#include <cstdio>
#include <cstdint>
#include <iostream>
#include <iomanip>
#include <string>
#include <queue>
#include <stack>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <utility>
#include <functional>
#include <cmath>
#include <climits>
#include <bitset>

// macros
#define ll long long int
#define pb push_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a); i<(b);++i)
#define rep(i, n) FOR(i, 0, n)

using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;

// solve

int main(int argc, char const* argv[])
{
  int n;
  cin >> n;
  rep(i, n){
    double x[3], y[3];
    rep(j, 3){
      cin >> x[j] >> y[j];
    }
    int tmp = 0;
    while(x[tmp] == x[(tmp + 1) % 3] || x[tmp] == x[(tmp + 2) % 3]){
      swap(x[0], x[tmp]);
      swap(y[0], y[tmp]);
      tmp++;
    }
    double yc = (((x[0] - x[2]) * (x[0] * x[0] + y[0] * y[0] - x[1] * x[1] - y[1] * y[1])) \
                - ((x[0] - x[1]) * (x[0] * x[0] + y[0] * y[0] - x[2] * x[2] - y[2] * y[2]))) \
                / ((x[0] - x[2]) * (y[0] - y[1]) - (x[0] - x[1]) * (y[0] - y[2])) / 2.;
    double xc = (((y[0] - y[2]) * (x[0] * x[0] + y[0] * y[0] - x[1] * x[1] - y[1] * y[1])) \
                - ((y[0] - y[1]) * (x[0] * x[0] + y[0] * y[0] - x[2] * x[2] - y[2] * y[2]))) \
                / ((y[0] - y[2]) * (x[0] - x[1]) - (y[0] - y[1]) * (x[0] - x[2])) / 2.;
    cout << setprecision(3) << fixed;
    cout << xc << " " << yc << " ";
    cout << sqrt((x[0] - xc) * (x[0] - xc) + (y[0] - yc) * (y[0] - yc)) << endl;
  }
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

int main(){

  int n;
  double x1, y1, x2, y2, x3, y3;
  
  cin >> n;
  for(;n--;){
    cin >> 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 = sqrt(abs(x1-x)*abs(x1-x) + abs(y1-y)*abs(y1-y));

    printf("%.3lf %.3lf %.3lf\n", x, y, r);
    
  }
}