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<stdio.h>
#include<math.h>
#include<string>
#include<algorithm>
#include<utility>
using namespace std;
double zeti(double);
int main() {
    pair<double, double> a;
    pair<double, double> b;
    pair<double, double> c;
    pair<double, double> m1;
    pair<double, double> m2;
    double s1, s2,x1,y1,x2,y2,x,y,b1,b2;
    int n;
    scanf("%d", &n);
    for (int i = 0; i < n; ++i) {
        x = -1000.0;
        y = -1000.0;
        b1 = 0.0;
        b2 = 0.0;
        s1 = -1000.0;
        s2 = -1000.0;
        scanf("%lf%lf", &a.first, &a.second);
        scanf("%lf%lf", &b.first, &b.second);
        scanf("%lf%lf", &c.first, &c.second);
        m1.first = (a.first + b.first) / 2.0;
        m1.second = (a.second + b.second) / 2.0;
        m2.first = (b.first + c.first) / 2.0;
        m2.second = (b.second + c.second) / 2.0;
        //swap(m1.first, m1.second);
        //swap(m2.first, m2.second);
        //x is qual
        if (a.first - b.first == 0) {
            x = m1.second;
            b1 = m1.second;
            s1=0;
        }
        //y is qual
        else if (a.second - b.second == 0) {
            
            y = m1.first;
            
        }
        else {
            s1 = (a.second - b.second) / (a.first - b.first);
            s1 = 1.0 / s1*-1.0;
            b1 = m1.second - s1*m1.first;
        }
        //x is qual
        if (b.first - c.first == 0) {
            x = m2.second;
            b2 = m2.second;
            s2=0;
        }
        //y is qual
        else if (b.second - c.second == 0) {
            y = m2.first;
        }
        else {
            s2 = (c.second - b.second) / (c.first - b.first);
            s2 = 1.0 / s2*-1.0;
            b2 = m2.second - s2*m2.first;
        }
        double r,w,e;
        if(x!=-1000.0){
            if(s1==-1000.0){
                y=s2*x+b2;
            }
            else{
                y=s1*x+b2;
            }
            w = zeti(a.first - x);
            e = zeti(a.second - y);
            r = sqrt(w*w+e*e);
            printf("%.3lf %.3lf %.3lf\n", x, y,r);
            return 0;
            
        }
        if(y!=-1000.0){
            cout<<"b"<<endl;
            if(s1==-1000.0){
                x=(y-b2)/s2;
            }
            else{
                x=(y-b1)/s1;
            }
            w = zeti(a.first - x);
            e = zeti(a.second - y);
            r = sqrt(w*w+e*e);
            printf("%.3lf %.3lf %.3lf\n", x, y,r);
            return 0;
        }
        if (x == -1000.0) {
            x = (b1 - b2) / (s2 - s1);
        }
        if (y==-1000.0) {
            y = s2*x + b2;
        }
        
        
        w = zeti(a.first - x);
        e = zeti(a.second - y);
        r = sqrt(w*w+e*e);
        printf("%.3lf %.3lf %.3lf\n", x, y,r);
        
    }
    return 0;
}
double zeti(double center) {
    if (center < 0) {
        center *= -1;
    }
    return center;
}

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

using namespace std;

int main() {
    int n, a, b, c, d, e, f;
    double x1, x2, x3, y1, y2, y3, px, py, r;
    cin >> n;
    while (n--) {
        cin >> x1 >> x2 >> x3 >> y1 >> y2 >> y3;
        a = 2 * (x1 - x2);
        b = 2 * (y1 - y2);
        c = x1*x1 + y1*y1 - x2*x2 - y2*y2;
        d = 2 * (x1 - x3);
        e = 2 * (y1 - y3);
        f = x1*x1 + y1*y1 - x3*x3 - y3*y3;
        px = (c*e - b*f) / (a*e - b*d);
        py = (c*d - a*f) / (b*d - a*e);
        r = sqrt(pow(px - x1, 2) + pow(py - y1, 2));
        printf("%.3f %.3f %.3f\n", px, py, r);
    }
    
    return 0;
}

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;

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);
    r = sqrt(r);
    int tr = (int)(10000 * r);
    if (tr % 10 >= 5) {
      tr += 10;
    }
    tr /= 10;
    r = (float)tr / 1000.0f;
    
    printf("%.3f\n", r);
  }  
  return 0;
}