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 <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
// ------ Classes ------ //
class Point {
public:
	long double px, py;
	Point() : px(0), py(0) {};
	Point(long double px_, long double py_) : px(px_), py(py_) {};
	bool operator==(const Point& p) const { return px == p.px && py == p.py; }
	bool operator!=(const Point& p) const { return px != p.px || py != p.py; }
	bool operator<(const Point& p) const { return px < p.px ? true : (px == p.px && py < p.py); }
	bool operator>(const Point& p) const { return px > p.px ? true : (px == p.px && py > p.py); }
	bool operator<=(const Point& p) const { return !(Point(px, py) > p); }
	bool operator>=(const Point& p) const { return !(Point(px, py) < p); }
	Point operator+(const Point& p) const { return Point(px + p.px, py + p.py); }
	Point operator-(const Point& p) const { return Point(px - p.px, py - p.py); }
	Point operator/(long double d) const { return Point(px / d, py / d); }
	friend Point operator*(const Point p, long double d) { return Point(p.px * d, p.py * d); }
	friend Point operator*(long double d, const Point& p) { return p * d; }
	Point& operator+=(const Point& p1) { px += p1.px, py += p1.py; return *this; }
	Point& operator-=(const Point& p1) { px -= p1.px, py -= p1.py; return *this; }
	Point& operator*=(long double d) { px *= d, py *= d; return *this; }
	Point& operator/=(long double d) { px /= d, py /= d; return *this; }
};
class Circle {
public:
	Point p; long double r;
	Circle() : p(Point()), r(0.0L) {};
	Circle(Point p_) : p(p_), r(0.0L) {};
	Circle(Point p_, long double r_) : p(p_), r(r_) {};
	Circle(long double x_, long double y_) : p(Point(x_, y_)), r(0.0L) {};
	Circle(long double x_, long double y_, long double r_) : p(Point(x_, y_)), r(r_) {};
	bool operator==(const Circle& c) const { return p == c.p && r == c.r; }
	bool operator!=(const Circle& c) const { return !(Circle(p, r) == c); }
};
// ------ Functions ------ //
long double norm(const Point& a) { return a.px * a.px + a.py * a.py; }
long double abs(const Point& a) { return sqrtl(norm(a)); }
long double arg(const Point& a) { return atan2l(a.py, a.px); }
long double dot(const Point& a, const Point& b) { return a.px * b.px + a.py * b.py; }
long double crs(const Point& a, const Point& b) { return a.px * b.py - a.py * b.px; }
long double dst(const Point& a, const Point& b) { return abs(b - a); }
Point pol(long double r, long double d) { return Point(cosl(d) * r, sinl(d) * r); }
vector<Point> crp(Circle c1, Circle c2) {
	double d = abs(c1.p - c2.p);
	double a = acos((c1.r * c1.r + d * d - c2.r * c2.r) / (2 * c1.r * d));
	double t = arg(c2.p - c1.p);
	return{ c1.p + pol(c1.r, t + a), c1.p + pol(c1.r, t - a) };
}
// ------ Main ------ //
int n; Point p1, p2, p3;
int main() {
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> p1.px >> p1.py >> p2.px >> p2.py >> p3.px >> p3.py;
		long double d1 = dst(p1, p2), d2 = dst(p2, p3), d3 = dst(p3, p1);
		long double r = d1 * d2 * d3 / sqrtl((d1 + d2 + d3) * (d1 + d2 - d3) * (d2 + d3 - d1) * (d3 + d1 - d2));
		vector<Point> v = crp(Circle(p1, r), Circle(p2, r));
		for (int j = 0; j < v.size(); j++) {
			long double d4 = dst(v[j], p3);
			if (d4 - 1e-10 <= r && r <= d4 + 1e-10) {
				if (-0.00050002L <= v[j].px && v[j].px < 0.00049999L) v[j].px = 0.0L;
				if (-0.00050002L <= v[j].py && v[j].py < 0.00049999L) v[j].py = 0.0L;
				printf("%.3Lf %.3Lf %.3Lf\n", v[j].px, v[j].py, r); break;
			}
		}
	}
	return 0;
}

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

int main(){
	int n; cin >> n;
	while(n--){
	double x[3] = {};
	double y[3] = {};

	for(int i=0; i<3; i++)
		cin >> x[i] >> y[i];

	double a,b,c,d,e,f;
	a = x[1] - x[0];
	b = y[1] - y[0];
	c = x[2] - x[0];
	d = y[2] - y[0];
	e = (-pow(x[0],2.0)+pow(x[1],2.0)-pow(y[0],2.0)+pow(y[1],2.0))/2;
	f = (-pow(x[0],2.0)+pow(x[2],2.0)-pow(y[0],2.0)+pow(y[2],2.0))/2;

	double Xo = (d*e-b*f)/(a*d-b*c);
	double Yo;
	if(b != 0)
		Yo=(e-a*Xo)/b;
	else
		Yo=(f-c*Xo)/d;

	cout << fixed;
	cout << setprecision(3) << Xo << " " << Yo << " " << sqrt(pow(Xo-x[0],2.0)+pow(Yo-y[0],2.0)) << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<complex>
#include<vector>
#include<cmath>
#include<algorithm>
#include<cstdio>

using namespace std;

#define rep(i, n) for (int i = 0; i < int(n); ++i)

#define X real()
#define Y imag()

typedef long double D;
typedef complex<D> P;

// 内積
D dot(P a, P b) {return a.X * b.X + a.Y * b.Y;}
// 外積
D cross(P a, P b) {return a.X * b.Y - a.Y * b.X;}

D aTri(D a, D b, D c){
  D s = (a + b + c) / 2;
  return sqrt(max(s * (s - a) * (s - b) * (s - c), (D)0));
}

P circumscribedCircleCenter(P a, P b, P c) {
  P bc = c - b, ca = a - c, ab = b - a;
  return (b + c - bc * P(0, 1) * dot(ca, ab) / cross(ca, ab)) / (D)2;
}

D circumscribedCircleRadius(D a, D b, D c) {return a * b * c / (4 * aTri(a, b, c));}

int main() {
  int n;
  cin >> n;
  rep (i, n) {
    P a, b, c;
    cin >> a.X >> a.Y >> b.X >> b.Y >> c.X >> c.Y;
    P p = circumscribedCircleCenter(a, b, c);
    D r = circumscribedCircleRadius(abs(a - b), abs(b - c), abs(c - a));
    printf("%.3Lf %.3Lf %.3Lf\n", p.X, p.Y, r);
  }
  return 0;
}