Go: package main

import (
	"fmt"
	"math"
)

func main() {
	n := scanInt()
	for i := 0; i < n; i++ {
		circleTriangle()
	}
}

func scanInt() int {
	var n int
	fmt.Scanf("%d", &n)
	return n
}

func circleTriangle() {
	var x1, y1, x2, y2, x3, y3 float64
	fmt.Scanf("%f%f%f%f%f%f", &x1, &y1, &x2, &y2, &x3, &y3)
	a := 2 * (x1 - x2)
	b := 2 * (y1 - y2)
	c := 2 * (x1 - x3)
	d := 2 * (y1 - y3)
	e := x1*x1 - x2*x2 + y1*y1 - y2*y2
	f := x1*x1 - x3*x3 + y1*y1 - y3*y3

	px := (d*e - b*f) / (a*d - b*c)
	py := (a*f - c*e) / (a*d - b*c)

	r := math.Sqrt((x1-px)*(x1-px) + (y1-py)*(y1-py))
	fmt.Printf("%.3f %.3f %.3f\n", px, py, r)
}

C++: #include <bits/stdc++.h>
using namespace std;
using uint = unsigned int;
using lint = long long int;
using ulint = unsigned long long int;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;
template<class T, class U> void assign(V<T>& v, int n, const U& a) { v.assign(n, a); }
template<class T, class... Args> void assign(V<T>& v, int n, const Args&... args) { v.resize(n); for (auto&& e : v) assign(e, args...); }


using P = complex<long double>;

int main() {
  cin.tie(nullptr); ios_base::sync_with_stdio(false);
  int n; cin >> n;
  for (int _ = 0; _ < n; ++_) {
    V<P> p(3);
    for (int i = 0; i < 3; ++i) {
      long double x, y; cin >> x >> y;
      p[i] = {x, y};
    }
    P numer = 0;
    for (int i = 0; i < 3; ++i) {
      numer += p[i] * (norm(p[(i + 2) % 3]) - norm(p[(i + 1) % 3]));
    }
    P denom = 0;
    for (int i = 0; i < 3; ++i) {
      denom += p[i] * conj(p[(i + 2) % 3] - p[(i + 1) % 3]);
    }
    P r = numer / denom;
    cout << fixed << setprecision(3) << real(r) << ' ' << imag(r) << ' ' << abs(p[0] - r) << '\n';
  }
}

C++: #include<cstdio>
#include<cmath>
using namespace std;
double x1,Y1,x2,y2,x3,y3;
double px, py,r;
int main()
{
    int n;
    scanf("%d",&n);
    while(n--)
    {
        scanf("%lf%lf%lf%lf%lf%lf",&x1,&Y1,&x2,&y2,&x3,&y3);
        double A1 =(x2*x2+y2*y2-x1*x1-Y1*Y1)*(y3-Y1);
        double B1 =(x3*x3+y3*y3-x1*x1-Y1*Y1)*(y2-Y1);
        double A2 =(x3*x3+y3*y3-x1*x1-Y1*Y1)*(x2-x1);
        double B2 =(x2*x2+y2*y2-x1*x1-Y1*Y1)*(x3-x1);
        double D = (x2-x1)*(y3-Y1)-(y2-Y1)*(x3-x1);
         px=(A1-B1)/D/2;
         py=(A2-B2)/D/2;
         r = sqrt((px-x1)*(px-x1)+(py-Y1)*(py-Y1));
        printf("%.3f %.3f %.3f\n",px,py,r);
    }
    return 0;
}

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

int main ()
{
    int num;
    double x1,y1,x2,y2,x3,y3;

    cin >> num;

    for( int i = 0; i < num; i++ )
    {
        double px = 0, py = 0, r = 0;
        double a, b, c, d, e, f, g;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

        px = ((x1*x1-x3*x3+y1*y1-y3*y3)*(y2-y1)-(x1*x1-x2*x2+y1*y1-y2*y2)*(y3-y1)) / (2*(x2-x1)*(y3-y1)-2*(x3-x1)*(y2-y1));
        
        /*
        a = (x1*x1-x3*x3+y1*y1-y3*y3);
        b = (y2-y1);
        c = (x1*x1-x2*x2+y1*y1-y2*y2);
        d = (y3-y1);
        e = 2*(x2-x1)*(y3-y1);
        f = 2*(x3-x1)*(y2-y1);

        cout << "a: " << a << " b: " << b << " c: " << c << " d: " << d << " e: " << e << " f: " << f << endl;
        g = (a*b-c*d) / (e-f);
        cout << "g: " << g << endl;
        */

        py = ((2*(x1-x2)*px+x2*x2-x1*x1+y2*y2-y1*y1)) / (2*(y2-y1));
        //r = (px-x1)*(px-x1) + (py-y1)*(py-y1);

        cout << fixed;
        cout << setprecision(3) << px << " " << py << " " << sqrt(r) << endl;
    }

    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
typedef pair<int,string> P;

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

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

using db = double;

struct poi {
	db x, y;
	void r() { cin >> x >> y; }
	poi operator -(poi p) {
		return {x - p.x, y - p.y};
	}
	poi operator /(db d) {
		return {x / d, y / d};
	}
	db cross(poi p) {
		return x * p.y - y * p.x;
	}
	db len2() {
		return x * x + y * y;
	}
};

poi circum(poi a, poi b, poi c) {
	poi ba = b - a, ca = c - a;
	db lb = ba.len2(), lc = ca.len2(), s = ba.cross(ca);
	return a - poi{ba.y * lc - ca.y * lb,
		ca.x * lb - ba.x * lc} / s / 2;
}

int main() {
	cout << fixed << setprecision(3);
	ios :: sync_with_stdio(false);
	int t; cin >> t;
	while(t --) {
		poi a, b, c;
		a.r(); b.r(); c.r();
		poi o = circum(a, b, c);
		cout << o.x << ' ' << o.y << ' ' << sqrt((o - a).len2()) << '\n';
	}
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
int main(){
    int a;
    scanf("%d",&a);
    while(a--){
        double x1,y1,x2,y2,x3,y3;
        scanf("%lf%lf%lf%lf%lf%lf",&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=hypot(x-x1,y-y1);
        printf("%.3lf %.3lf %.3lf\n",x,y,r);
    }
}

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

using namespace std;

int main()
{
    int n;
    cin >> n;
    double x1,y1,x2,y2,x3,y3;
    while(n>0)
    {
        cin >> x1 >> y1 >>x2 >> y2 >> x3 >> y3;
        double s,v1x,v1y,v2x,v2y;
        v1x = y2-y1;
        v1y = x1-x2;

        v2x = y3-y1;
        v2y = x1-x3;
        double detA = v1x*v2y-v1y*v2x;
        double hx = (x2-x1)/2;
        double hy = (y2-y1)/2;
        s = (v2y*hx -v1y*hy)/detA;
        double ox,oy;
        ox = hx + s*v1x;
        oy = hy + s*v1y;
        if(-0.0005<ox&&ox<0)ox = 0;
        if(-0.0005<oy&&oy<0)oy = 0;
        double r = sqrt((x1-ox)*(x1-ox)+(y1-oy)*(y1-oy));
        printf("%.3f %.3f %.3f\n",ox,oy,r);
        --n;
    }
    return 0;
}

C++: #define _USE_MATH_DEFINES

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <algorithm>
#include <utility>
#include <vector>
#include <numeric>

using namespace std;

double distance(double x0, double y0, double x1, double y1)
{
	double x = x1 - x0;
	double y = y1 - y0;

	return sqrt(x * x + y * y);
}

int main()
{
	int n;
	for (cin >> n; n--;)
	{
		vector<double> x(3);
		vector<double> y(3);

		for (int i = 0; i < 3; i++)
		{
			cin >> x.at(i) >> y.at(i);
		}

		double a = distance(x.at(1), y.at(1), x.at(2), y.at(2));
		double b = distance(x.at(2), y.at(2), x.at(0), y.at(0));
		double c = distance(x.at(0), y.at(0), x.at(1), y.at(1));

		double cosA = (b * b + c * c - a * a) / (2 * b * c);
		double sinA = sqrt(1 - cosA * cosA);
		double r = a / (2 * sinA);

		double cx = r * cosA;
		double cy = r * sinA;

		cout << fixed << setprecision(3);
		cout << cx << " " << cy << " " << r << endl;
	}
}