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 <iomanip>
#include <math.h>
using namespace std;

int main()
{
	double x1, y1, x2, y2, x3, y3;
	double a1, b1, c1, a2, b2, c2;
	double xp, yp, r;
	int loop;
	
	cin >> loop;
	
	while(loop>0){
		while(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;
		
			xp = (b1*c2-b2*c1)/(a1*b2-a2*b1);
			yp = (c1*a2-c2*a1)/(a1*b2-a2*b1);
		
			r = sqrt(pow((x1-xp), 2) + pow((y1-yp), 2));
				
			cout << fixed << setprecision(3) << xp << " " << yp << " " << r << endl;
			
			--loop;
			if(loop==0){
				break;
			}
		}
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<cmath>
using namespace std;
long double ff(long double a){
  
  a*=10000.0;
  //  cout<<a<<endl;
  long long c = a;
  if(c%10>4)c+=10;
  c/=10;
  return (long double)c/1000.0;
}



long double ax,ay,bx,by,cx,cy,ox,oy,dx,dy,ex,ey;
//ack はACの傾き  eos はEOの切片
long double abk,ack,dok,eok,eos,dos,eps=0.0;
int n;
int main(){

  cin>>n;
  for(int i=0;i<n;i++){
    cin>>ax>>ay>>bx>>by>>cx>>cy;
    //ABの中点をD,  ACの中点をE 外接円の中心をO とする
    dx=(ax+bx)/2.0;
    dy=(ay+by)/2.0;
    ex=(ax+cx)/2.0;
    ey=(ay+cy)/2.0;
    
    if(ax==bx){
      abk=(ay-by)/0.000000000001;
    }else{
      abk=(ay-by)/(ax-bx);
    }
    if(abk==0.0)abk=0.000000000001;
    dok=-1.0/abk;
    dos=dy-dx*dok;
    
    
    if(ax==cx){
      ack=(ay-cy)/0.000000000001;
    }else{
      ack=(ay-cy)/(ax-cx);
    }
    if(ack==0.0)ack=0.000000000001;
    eok=-1.0/ack;
    eos=ey-ex*eok;
    
    //Y  =  dok X  + dos
    //Y  =  eok X  + eos
    //dok X + dos = eok X + eos
    
    long double Y,X;
    if(dok-eok==0.0){
      X=(eos-dos)/0.000000000001;
    }else{
      X=(eos-dos)/(dok-eok);
    }
    Y= dok * X + dos;
    
    printf("%.3Lf %.3Lf ",X,Y);
    //cout<<ax<<' '<<X<<' '<<ay<<' '<<Y<<endl;
    printf("%.3Lf\n" ,  sqrt( pow(ax-X,2) + pow(ay-Y,2) ));

    
    
  }
  return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>

#define int64 long long int

using namespace std;

double round(double x){
	if(x > 0) return floor(x + 0.5);
	else return -1.0 * floor(fabs(x) + 0.5);
}

int main()
{
	int n;
	double x1,y1,x2,y2,x3,y3,x,y,r;
	double det,a1,b1,a2,b2,c1,c2;
	cin >> n;
	for(int i=0; i<n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a1 = 2 * (x1 - x2), b1 = 2 * (y1 - y2);
		a2 = 2 * (x2 - x3), b2 = 2 * (y2 - y3);
		det = a1 * b2 - a2 * b1;
		c1 = (x1*x1 - x2*x2) + (y1*y1 - y2*y2);
		c2 = (x2*x2 - x3*x3) + (y2*y2 - y3*y3);
		x = (a1 * c1 + b1 * c2) / det;
		y = (a2 * c1 + b2 * c2) / det;
		r = sqrt((x - x1)*(x - x1) + (y - y1)*(y - y1));
		x *= 1000, y *= 1000, r *= 1000;
		x = round(x), y = round(y), r = round(r);
		x /= 1000, y /= 1000, r /= 1000;
		if(fabs(x) < 1e-12) x = 0.0;
		if(fabs(y) < 1e-12) y = 0.0;
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}

C++: #include <stdio.h>
#include <math.h>
#define NOT_E 200

double cequal(double *a,double *b,double *c,double *aa,double *bb,double *cc);
double equal(double a,double b,double c,double aa,double bb,double cc);
double answer(double x1,double x2,double y1,double y2,double *a,double *b);
int swap(double *a,double *b);

int main(){
	int i,cnt;
	double x1,x2,x3,y1,y2,y3,xp,yp/*¦*/,a1,a2,b1,b2,xm1,ym1,xm2,ym2,r;
	
	scanf("%d",&cnt);
	for(i=0;i<cnt;i++){
		
		scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
		
		xp=equal(y1,y2,y3,x1,x2,x3);
		yp=equal(x1,x2,x3,y1,y2,y3);
		
		
		if (xp==NOT_E || yp==NOT_E){
			if(xp==NOT_E){
				cequal(&x1,&x2,&x3,&y1,&y2,&y3);
			}else if(yp==NOT_E){
				cequal(&y1,&y2,&y3,&x1,&x2,&x3);
			}
			answer(x3,x2,y3,y2,&a1,&b1);
			answer(x3,x1,y3,y1,&a2,&b2);
			xp=(b2-b1)/(a1-a2);
			yp=a1*xp+b1;
			
		}
		
		if(xp==x1){
			r=abs(xp-x1);
		}else if(yp==y2){
			r=abs(yp-y1);
		}else{
			r=sqrt((xp-x1)*(xp-x1)+(yp-y1)*(yp-y1));
		}
		
		printf("%.3f %.3f %.3f\n",xp,yp,r);

	}
	return 0;
}
			
			
			
			

double cequal(double *a,double *b,double *c,double *aa,double *bb,double *cc){	/*a1=a2,a3ÆÈéæ¤ÉÀÑÖ¦é*/
	if(*a==*b){
	}else if (*b==*c){
		swap(a,c);
		swap(aa,cc);
	}else if (*c==*a){
		swap(b,c);
		swap(bb,cc);
	}else{
		return NOT_E;
	}
	
	return (*a+*b)/2;
}


double equal(double a,double b,double c,double aa,double bb,double cc){	/*ÀÑÖ¦È¢*/
	if(a==b){
	}else if (b==c){
		swap(&a,&c);
		swap(&aa,&cc);
	}else if (c==a){
		swap(&b,&c);
		swap(&bb,&cc);
	}else{
		return NOT_E;
	}
	return (aa+bb)/2;
}



double answer(double x1,double x2,double y1,double y2,double *a,double *b){	/*(x1,y1)Æ(x2,y2)Ì¼ñªüy=ax+bÌa,bð|C^ÅÔ·*/
	double xm,ym;
	*a=(y2-y1)/(x1-x2);
	xm=(x2+x1)/2;
	ym=(x2+y1)/2;
	*b=ym-*a*xm;
	return 0;
}

int swap(double *a,double *b){
	double temp;
	temp=*a;
	*a=*b;
	*b=*a;
	return 0;
}
	/*
	a2=(y1-y3)/(x3-x1);
	xm2=(x1+x3)/2;
	ym2=(x1+y3)/2;
	b2=ym2-a3*xm2;*/

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

int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    double x1, y1, x2, y2, x3, y3, px, py, r;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    py = ((x3 - x1) * (x1 * x1 + y1 * y1 - x2 * x2 - y2 * y2) - (x2 - x1) * (x1 * x1 + y1 * y1 - x3 * x3 - y3 * y3)) /
         (2 * (x3 - x1) * (y1 - y2) - 2 * (x2 - x1) * (y1 - y3));
    if (x2 - x1 != 0) {
      px = (2 * (y1 - y2) * py - x1 * x1 - y1 * y1 + x2 * x2 + y2 * y2) / (2 * (x2 - x1));
    } else {
      px = (2 * (y1 - y3) * py - x1 * x1 - y1 * y1 + x3 * x3 + y3 * y3) / (2 * (x3 - x1));
    }
    r = sqrt((px - x1) * (px - x1) + (py - y1) * (py - y1));
    cout << fixed << setprecision(3) << px << " " << py << " " << r << endl;
  }
  return 0;
}