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 <cstdio>
#include <cmath>
#include <utility>
using namespace std;


class vec{
public:
	double x;
	double y;
	double normEXP2() const{
		return x*x + y*y;
	}
	vec& operator+(const vec& v){
		x += v.x;
		y += v.y;
		return *this;
	}
	vec& operator-(const vec& v){
		x -= v.x;
		y -= v.y;
		return *this;
	}
	vec& operator*(double scala){
		x *= scala;
		y *= scala;
		return *this;
	}
	double operator*(const vec& v) const{
		return x*v.x + y*v.y;
	}
};


istream& operator>>(istream& i, vec& v){
	i >> v.x >> v.y;
	return i;
}


pair<double, double> cramer(const vec& a, const vec& b){
	pair<double, double> solve;
	solve.first = (2 * a.normEXP2()*b.normEXP2() - 2 * b.normEXP2()*(a*b)) / (4 * a.normEXP2()*b.normEXP2() - 4 * ((a*b)*(a*b)));
	solve.second = (2 * a.normEXP2()*b.normEXP2() - 2 * a.normEXP2()*(a*b)) / (4 * a.normEXP2()*b.normEXP2() - 4 * ((a*b)*(a*b)));

	return solve;
}


int main(){
	int n;
	vec v1, v2, v3;
	vec b, c;
	vec solveVec;
	pair<double, double> solve;
	double x, y;
	double r;

	cin >> n;
	for (int i = 0; i < n; i++){
		cin >> v1 >> v2 >> v3;
		b = v2 - v1;
		c = v3 - v1;
		
		solve = cramer(b, c);
		solveVec = (b*solve.first) + (c*solve.second);

		solveVec = v1 + solveVec;
		r = sqrt(solveVec.normEXP2());

		y = (double)((int)((solveVec.y * 1000) + 0.5)) / 1000;
		r = (double)((int)((r * 1000) + 0.5)) / 1000;
		x = (double)((int)((solveVec.x * 1000) + 0.5)) / 1000;

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

	return 0;
}

C++: int main(void){

  int n;
  cin >> n;

  for(int i = 0;i < n;i++){

    double x1,y1,x2,y2,x3,y3;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

    double a = sqrt(pow(x2-x1,2) + pow(y2-y1,2));
    double b = sqrt(pow(x3-x2,2) + pow(y3-y2,2));
    double c = sqrt(pow(x3-x1,2) + pow(y3-y1,2));

    double r = a*b*c / sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));

    double A = pow(x2-x1,2) + pow(y2-y1,2);
    double B = pow(x3-x2,2) + pow(y3-y2,2);
    double C = pow(x3-x1,2) + pow(y3-y1,2);
    double s = 4*((x2-x1)*(y3-y1) -(x3-x1)*(y2-y1));

    double x = A*(B+C-A) / s;
    double y = B*(C+A-B) / s;

    printf("%.3f %.3f %.3f\n");

  }




  return 0;
}

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

using namespace std;

long double sisya(long double a) {
	a *= 1000.0;
	if (a-(int)a >= 0.5) {
		a += 1.0;
	}
	a /= 1000.0;
	return a; 
}

int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		long double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3; 
		long double a, b, c;
		a = sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2));
		b = sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
		c = sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
		long double s = (a+b+c)/2.0;
		long double S = sqrt(s*(s-a)*(s-b)*(s-c));
		long double p1, p2, pr;
		p1 = (a*a*(b*b+c*c-a*a)*x1+b*b*(c*c+a*a-b*b)*x2+c*c*(a*a+b*b-c*c)*x3)/(16.0*S*S);
		p2 = (a*a*(b*b+c*c-a*a)*y1+b*b*(c*c+a*a-b*b)*y2+c*c*(a*a+b*b-c*c)*y3)/(16.0*S*S);
		pr = (a*b*c)/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
		p1 = sisya(p1);
		p2 = sisya(p2);
		pr = sisya(pr);
		printf("%.3f %.3f %.3f\n", p1, p2, pr);
	}
	return 0;
}

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

#define N 10000
#define INF 200000000

int main()
{
    int n;
    vector<double> x(3), y(3), x2(3), y2(3);
    cin >> n;

    for(int p=0; p<n; p++)
    {
        cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2];
        for(int i=0; i<3; i++) { x2[i] = x[i]*x[i]; y2[i] = y[i]*y[i]; }

        double a, b, c, a2, b2, c2;
        a2 = abs((x[2]-x[1])*(x[2]-x[1]) + (y[2]-y[1])*(y[2]-y[1]));
        b2 = abs((x[0]-x[2])*(x[0]-x[2]) + (y[0]-y[2])*(y[0]-y[2]));
        c2 = abs((x[1]-x[0])*(x[1]-x[0]) + (y[1]-y[0])*(y[1]-y[0]));
        a = sqrt(a2); b = sqrt(b2); c = sqrt(c2);

        double s, S2;
        s = (a + b + c) / 2;
        S2 = s * (s-a) * (s-b) * (s-c);

        double cx, cy;
        cx = (a2*(b2+c2-a2)*x[0] + b2*(c2+a2-b2)*x[1] + c2*(a2+b2-c2)*x[2]) / (16 * S2);
        cy = (a2*(b2+c2-a2)*y[0] + b2*(c2+a2-b2)*y[1] + c2*(a2+b2-c2)*y[2]) / (16 * S2);

        cout << setprecision(3) << fixed <<
            cx << " " << cy << " " << sqrt(abs((cx-x[0])*(cx-x[0])+(cy-y[0])*(cy-y[0]))) << endl;
    }
}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
#define MAX_N 20
using namespace std;

void swap(double &x,double &y){
	double t;
	t=x;
	x=y;
	y=t;
}

double cut4(double x){
	double temp=x*pow(10,3);
	temp=double(int(temp+0.5));
	return temp*pow(10,-3);
}

int main(){
	int n;
	double x1,x2,x3,y1,y2,y3,px,py;
	double a,b,c,d,r;
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		if(x1==x2){
			swap(x2,x3);
			swap(y2,y3);
		}else if(x2==x3){
			swap(x1,x2);
			swap(y1,y2);
		}
		a=(y2-y1)/(x2-x1);
		c=(y3-y2)/(x3-x2);
		if(a&&c){
			a=-1/a;
			c=-1/c;
			b=(y1+y2)/2-a*(x1+x2)/2;
			d=(y2+y3)/2-c*(x2+x3)/2;
			px=(b-d)/(c-a);
			py=(b*c-a*d)/(c-a);
		}else if(!a){
			c=-1/c;
			d=(y2+y3)/2-c*(x2+x3)/2;
			px=(x1+x2)/2;
			py=c*(x1+x2)/2+d;
		}else if(!c){
			a=-1/a;
			b=(y1+y2)/2-a*(x1+x2)/2;
			px=(x2+x3)/2;
			py=a*(x2+x3)/2+b;
		}

		r=sqrt((px-x1)*(px-x1)+(py-y1)*(py-y1));
		px=(px==0?0.000:px);
		py=(py==0?0.000:px);
		printf("%.3lf %.3lf %.3lf\n",cut4(px),cut4(py),cut4(r));
	}
	return 0;
}

C++: #include<iostream>
#include<math.h>

using namespace std;

int main(){
	int n;
	double x1, y1, x2, y2, x3, y3;
	double x, y, r;
	double a1, a2, b1, b2, c1, c2;

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

		a1 = 2 * (x2 - x1);
		b1 = 2 * (y2 - y1);
		c1 = x1*x1 - x2*x2 + y1*y1 - y2*y2;

		a2 = 2 * (x3 - x1);
		b2 = 2 * (y3 - y1);
		c2 = x1*x1 - x3*x3 + y1*y1 - y3*y3;

		x = (b1*c2 - b2*c1) / (a1*b2 - a2*b1);
		y = (c1*a2 - c2*a1) / (a1*b2 - a2*b1);

		r = hypot(x - x1, y - y1);

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

	return 0;
}

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

int main()
{
   int n;
   cin>>n;
   while(n--){
       double x1,x2,x3,y1,y2,y3;
       double px,py,r;
       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 c = (x2-x1)*(y3-y1)-(y2-y1)*(x3-x1);
        px=(a1-b1)/c/2;
        py=(a2-b2)/c/2;
        r = sqrt((px-x1)*(px-x1)+(py-y1)*(py-y1));
        printf("%.3f %.3f %.3f\n",px,py,r);
   }
    return 0;
}