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>

using namespace std;

#define MAX 256

float LOSER(float x){
	if((int)(x*10000)%10 > 4)return x + 0.001;
	return x;
}

int main(void){
	int n,i = 0;
	float x1,y1,x2,y2,x3,y3;
	float px[MAX],py[MAX],r[MAX];
	
	cin >> n;
	while(i < n){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		px[i] = ((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[i] = ((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));
		r[i] = sqrt((px[i]-x1)*(px[i]-x1)+(py[i]-y1)*(py[i]-y1));
		px[i]=LOSER(px[i]);
		py[i]=LOSER(py[i]);
		r[i]=LOSER(r[i]);
		i++;
		printf("%.3f %.3f %.3f",px[i],py[i],r[i]);
	}

	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#define double long double
using namespace std;

double marume( double x ){
	long long t = x * 1000 + ((x > 0) ? 1 : -1) * 0.5;
	return t/1000.0;
}

int main(){
	int n;
	double tx, ty, x1, y1, x2, y2;
	
	cin >> n;
	while(n--){
		cin >> tx >> ty >> x1 >> y1 >> x2 >> y2;
		x1 -= tx; y1 -= ty;
		x2 -= tx; y2 -= ty;
		
		double dist1 = x1 * x1 + y1 * y1;
		double dist2 = x2 * x2 + y2 * y2;
		x1 *= 2; y1 *= 2;
		x2 *= 2; y2 *= 2;
		
		double detA = x1 * y2 - y1 * x2;
		double detX = dist1 * y2 - y1 * dist2;
		double detY = x1 * dist2 - dist1 * x2;
		double x = detX / detA; //if(x < 0){ x -= 1e-5; } else { x += 1e-5; }
		double y = detY / detA; //if(y < 0){ y -= 1e-5; } else { y += 1e-5; }
		double r = hypot(x, y) + 1e-7;
		
		x += tx;
		y += ty;
		
		//x = marume(x);
		//y = marume(y);
		//r = marume(r);
		printf("%.3Lf %.3Lf %.3Lf\n", x, y, r);
	}
	return 0;
}

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

int main(){
	int n;
	double x1, y1, x2, y2, x3, y3, m1, m2, m3, b1, b2, b3, px, py, r;
	double d[2], e[2], f[2];
	cin >> n;
	for(int i = 0; i < n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		d[0] = (x2 + x3) / 2;
		d[1] = (y2 + y3) / 2;
		e[0] = (x3 + x1) / 2;
		e[1] = (y3 + y1) / 2;
		f[0] = (x1 + x2) / 2;
		f[1] = (y1 + y2) / 2;
		m1 = (y3 - y2) / (x3 - x2);
		m2 = (y1 - y3) / (x1 - x3);
		m3 = (y2 - y1) / (x2 - x1);
		b1 = d[1] + d[0] / m1;
		b2 = e[1] + e[0] / m2;
		b3 = f[1] + f[0] / m3;
		px = -(b1 - b2) / ((1 / m2) - (1 / m1));
		py = -(1 / m1) * px + b1;
		r = (double)sqrt((x1 - px) * (x1 - px) + (y1 - py) * (y1 - py));
		px = round(10000 * px) / 10000;
		py = round(10000 * py) / 10000;
		r = round(10000 * r) / 10000;
		cout << fixed << setprecision(3) << px << " " <<  py << " " << r << endl;
	}
	return 0;
}

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

typedef struct
{
  double x, y;
} Point;

int main()
{
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) {
    Point A, B, C;
    scanf("%lf %lf %lf %lf %lf %lf", &A.x, &A.y, &B.x, &B.y, &C.x, &C.y);

    double a = sqrt((B.x-C.x)*(B.x-C.x) + (B.y-C.y)*(B.y-C.y));
    double b = sqrt((C.x-A.x)*(C.x-A.x) + (C.y-A.y)*(C.y-A.y));
    double c = sqrt((A.x-B.x)*(A.x-B.x) + (A.y-B.y)*(A.y-B.y));

    double angleA = acos((b*b+c*c-a*a) / (2*b*c));
    double angleB = acos((c*c+a*a-b*b) / (2*c*a));
    double angleC = acos((a*a+b*b-c*c) / (2*a*b));

    double sin2A = sin(2 * angleA);
    double sin2B = sin(2 * angleB);
    double sin2C = sin(2 * angleC);

    double x = (A.x*sin2A + B.x*sin2B + C.x*sin2C) / (sin2A+sin2B+sin2C);
    double y = (A.y*sin2A + B.y*sin2B + C.y*sin2C) / (sin2A+sin2B+sin2C);
    double r = (a / (2 * sin(angleA)));

    printf("%.3f %.3f %.3f", round(x*1000.0)/1000.0, round(y*1000.0)/1000.0, round(r*1000.0)/1000.0);
  }
  return 0;
}

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

int main()
{
	int n; cin>>n;
	while(n--)
	{
		double x1,y1,x2,y2,x3,y3;
		cin>>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=sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1));

		x=floor(x*1000+0.5)/1000;
		y=floor(y*1000+0.5)/1000;
		r=floor(r*1000+0.5)/1000;

		printf("%.3lf %.3lf %.3lf\n",x,y,r);
	}
	return 0;
}

C++:  #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 
 using namespace std;
 
 int main()
 {
   int n;
   cin>>n;
   while(n--)
   {
     double x1,y1,x2,y2,x3,y3;
     cin>>x1>>y1>>x2>>y2>>x3>>y3;
     double x=((y2-y1)*(y3*y3-y1*y1+x3*x3-x1*x1)-(y3-y1)*(y2*y2-y1*y1+x2*x2-x1*x1))/(2*(x3-x1)*(y2-y1)-2*((x2-x1)*(y3-y1)));  
     double y=((x2-x1)*(x3*x3-x1*x1+y3*y3-y1*y1)-(x3-x1)*(x2*x2-x1*x1+y2*y2-y1*y1))/(2*(y3-y1)*(x2-x1)-2*((y2-y1)*(x3-x1))); 
     double R=sqrt((x1-x)*(x1-x)+(y1-y)*(y1-y));
     printf("%.3lf %.3lf %.3lf\n",x,y,R);      
   }
   //while(1);      
   return 0;    
 }