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 <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) {
				int lx = (v[j].px + 0.00049999L) * 1000;
				int ly = (v[j].py + 0.00049999L) * 1000;
				printf("%.3Lf %.3Lf %.3Lf\n", lx / 1000.0L, ly / 1000.0L, r); break;
			}
		}
	}
	return 0;
}

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

int main(){

  double x1,y1,x2,y2,x3,y3;
  double px,py;
  int n;

  cin >> n;

  for(int i=0; i<n; i++){
    cin >> x1>>y1>>x2>>y2>>x3>>y3;
    px = ( (y1-y3)*(y1*y1-y2*y2 + x1*x1-x2*x2)-(y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3))/(2*(y1-y3)*(x1-x2)-2*(y1-y2)*(x1-x3));
    py = ( (x1-x3)*(x1*x1-x2*x2 + y1*y1-y2*y2)-(x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3))/(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));
    cout << px <<" "<< py <<" ";
    printf("%.3f\n", sqrt((px-x1)*(px-x1)+(py-y1)*(py-y1)));

  }


  return 0;
}

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

int main() {
	int n;
	cin >> n;
	for(int i=0;i<n;i++){
		double ax, ay, bx, by, cx, cy;
		cin >> ax >> ay >> bx >> by >> cx >> cy;
		double cs = (by - ay) / (bx - ax);//AB?????????
		double op = -1 / cs;

		double as = (cy - by) / (cx - bx);
		double on = -1 / as;

		double a, b, c, d, e, f;
		a = -1 * op;
		b = 1;
		c = -1 * (op*((ax + bx) / 2)) + ((ay + by) / 2);
		d = -1 * on;
		e = 1;
		f = -1 * (on*((bx + cx) / 2)) + ((by + cy) / 2);
		double any = ((c*d) - (a*f)) / ((b*d) - (a*e));
		double anx = (c - (b*any)) / a;
		double lng = sqrt((abs(anx - bx))*(abs(anx - bx)) + ((abs(any - by)*abs(any - by))));
		
		double aax = round(anx * 10000) / 10000;
		double aay = round(any * 10000) / 10000;
		double alng = round(lng * 10000) / 10000;
		cout << fixed << setprecision(3) << aax << ' ' << aay << ' ' << alng << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef double D;
typedef complex<D> P;
typedef vector<P> VP;
P minEnclosingCircle(const VP& ps) {
  P c;
  double move = 0.5;
  r(i,39) {  // 2^(-39-1) \approx 0.9e-12
    r(t,50) {
      D max = 0;
      int k = 0;
      r (j, ps.size()) if (max < norm(ps[j]-c)) {
        max = norm(ps[j]-c);
        k = j;
      }
      c += (ps[k]-c) * move;
    }
    move /= 2;
  }
  return c;
}
int main(){
    int n;
    cin>>n;
    r(i,n){
        VP p;
        D a,b;
        r(j,3){
            cin>>a>>b;
            p.push_back(P(a,b));
        }
        P t=minEnclosingCircle(p);
        D dis=max(abs(p[0]-t),max(abs(p[1]-t),abs(p[2]-t)));
        printf("%.3f %.3f %.3f\n",t.real(),t.imag(),dis);
    }
}

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

//0010
int main(){
	int n;
	while(cin >> n){
		double x1,y1,x2,y2,x3,y3,x,y,r;

		while(n--){
			cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

			if(x1!=x2){
				x=(x1+x2)/2;
			}else if(x2!=x3){
				x=(x2+x3)/2;
			}else if(x3!=x1){
				x=(x3+x1)/2;
			}
			if(y1!=y2){
				y=(y1+y2)/2;
			}else if(y2!=y3){
				y=(y2+y3)/2;
			}else if(y3!=y1){
				y=(y3+y1)/2;
			}

			r=sqrt(pow(x-x1,2)+pow(y-y1,2));
			cout.setf(ios::fixed);
			cout.precision(3);
			cout << x << " " << y << " " << r << endl;
		}
	}

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

int main(void)
{
    int n;
    double x1, x2, x3, y1, y2, y3;
    cin >> n;
    while (cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
        double a = 2 * (x1 - x2);
        double b = 2 * (y1 - y2);
        double c = x1*x1 - x2*x2 + y1*y1 - y2*y2;
        double d = 2 * (x1 - x3);
        double e = 2 * (y1 - y3);
        double f = x1*x1 - x3*x3 + y1*y1 - y3*y3;
        double p1 = (e * c - b * f) / (e * a - b * d);
        double p2 = (d * c - a * f) / (d * b - a * e);
        double r = sqrt(pow(x1 - p1, 2) + pow(y1 - p2, 2));
        cout << fixed << std::setprecision(3);
        cout << p1 << " " << p2 << " " << r;
    }
    return 0;
}