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 <stdio.h>
#include <math.h>

int main(void)
{
	float x1, x2, x3, y1, y2, y3;
	float xp, yp, r;
	float rs;
	int n;
	
	scanf("%d", &n);
	
	while (n){
		scanf("%f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3);
		
		yp = ((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));
		xp = ((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 = sqrt((x1 - xp) * (x1 - xp) + (y1 - yp) * (y1 - yp));
		
		printf("%.3f %.3f %.3f\n", xp, yp, r);
	}
	
	return (0);
}

C++: #include <iostream>
#include <vector>
//#include <math.h>
//#include <string>
#include <cmath>
#include <iomanip>

int main();
double round(double dSrc, int iLen);

int main(){
	int cnt = 0;		//データセットの個数
	double x1, y1, x2, y2, x3, y3;
	std::vector<double> p;		//外接円の中心のx座標
	std::vector<double> q;		//外接円の中心のy座標
	std::vector<double> r;		//外接円の半径
	
	std::cin >> cnt;
	for (int i = 0; i < cnt; i++){
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		p.push_back(((y1 - y3) * (pow(y1, 2) - pow(y2, 2) + pow(x1, 2) - pow(x2, 2)) - (y1 - y2) * (pow(y1, 2) - pow(y3, 2) + pow(x1, 2) - pow(x3, 2))) / (2 * (y1 - y3) * (x1 - x2) - 2 * (y1 - y2) * (x1 - x3)));
		q.push_back(((x1 - x3) * (pow(x1, 2) - pow(x2, 2) + pow(y1, 2) - pow(y2, 2)) - (x1 - x2) * (pow(x1, 2) - pow(x3, 2) + pow(y1, 2) - pow(y3, 2))) / (2 * (x1 - x3) * (y1 - y2) - 2 * (x1 - x2) * (y1 - y3)));
		r.push_back(sqrt((pow((p[i] - x1), 2) + pow((q[i] - y1), 2))));
		
		///*
		int temp = p[i];
		if (p[i] - temp != 0){
			p[i] = round(p[i], 3);
		}

		temp = q[i];
		if (q[i] - temp != 0){
			q[i] = round(q[i], 3);
		}
		
		temp = r[i];
		if (r[i] - temp != 0){
			r[i] = round(r[i], 3);
		}

		std::cout << std::fixed << std::setprecision(3) << p[i] << " ";
		std::cout << std::fixed << std::setprecision(3) << q[i] << " ";
		std::cout << std::fixed << std::setprecision(3) << r[i] << std::endl;
		//*/
	}
	/*
	for (int i = 0; i < cnt; i++){
		int temp = p[i];
		if (p[i] - temp != 0){
			p[i] = round(p[i], 3);
		}

		temp = q[i];
		if (q[i] - temp != 0){
			q[i] = round(q[i], 3);
		}
		
		temp = r[i];
		if (r[i] - temp != 0){
			r[i] = round(r[i], 3);
		}

		std::cout << std::fixed << std::setprecision(3) << p[i] << " ";
		std::cout << std::fixed << std::setprecision(3) << q[i] << " ";
		std::cout << std::fixed << std::setprecision(3) << r[i] << std::endl;
	}
	*/
	//while (1);
	return 0;
}

/**
* 指定桁で四捨五入を行う
* args:数値, 四捨五入する桁数
* return:結果
* 参考:http://oresi.hatenablog.com/entry/2014/06/23/【C%2CC%2B%2B】指定桁数で端数処理%EF%BC%88切り上げ、切り下げ
*/
double round(double dSrc, int iLen)
{
	double	dRet;

	dRet = dSrc * pow(10.0, iLen);
	dRet = (double)(int)(dRet + 0.5);

	return dRet * pow(10.0, -iLen);
}

C++: #include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull;
#define loop(i,a,b) for(int i=(a);i<ull(b);++i)
#define rep(i,n) loop(i,0,n)
#define all(a) (a).begin(), (a).end()

const double eps = 1e-10;
const double pi  = acos(-1.0);
const double inf = (int)1e8;

double distance(double x1, double y1, double x2, double y2){
    return sqrt(pow(abs(x1-x2), 2)+pow(abs(y1-y2), 2));
}

int main(){
    int n;
	cin>> n;
	rep(q, n){
	        double x1,y1,x2,y2,x3,y3;
	        cin >> x1 >>y1 >>x2 >>y2 >>x3 >>y3;
	        double a = distance(x1, y1, x2, y2), b = distance(x2, y2, x3, y3), c = distance(x3, y3, x1, y1);
	        double s = (a+b+c)/2;
	        double S = sqrt(s*(s-a)*(s-b)*(s-c));

			double r = a*b*c/4/S;

	        double a1 = 2*(x2-x1), b1 = 2*(y2-y1), c1 = x1*x1-x2*x2+y1*y1-y2*y2;
	        double a2 = 2*(x3-x1), b2 = 2*(y3-y1), 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);
	        printf("%.3f %.3f %.3f\n", x, y, r);
	}
}

C++: #include <iostream>
#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;
		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);
		//	http://www.h6.dion.ne.jp/~jpe02_tn/ki-5.html

		double r = sqrt((x1-x)*(x1-x)+(y1-y)*(y1-y));

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

}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int main(){

  double x1,x2,x3,y1,y2,y3;
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    double ax,ay,a,b,c,d,e,f,r;
    if(y2!=y1){
      a=-(x2-x1)/(y2-y1);
      b=-1;
      c=-a*(x1+x2)/2+(y1+y2)/2;
    }else{
      a=1;
      b=0;
      c=(x1+x2)/2;
    }
    if(y3!=y2){
      d=-(x3-x2)/(y3-y2);
      e=-1;
      f=-a*(x2+x3)/2+(y2+y3)/2;
    }else{
      d=1;
      e=0;
      f=(x2+x3)/2;
    }
    ax=(c*e-b*f)/(a*e-b*d);
    ay=(a*f-c*d)/(a*e-b*d);
    if(ax==0)
      ax=0;
    if(ay==0)
      ay=0;
    r=sqrt((x1-ax)*(x1-ax)+(y1-ay)*(y1-ay));
    if(ax*1000-(int)ax*1000>=0.5)
      ax+=0.001;
    if(ay*1000-(int)ay*1000>=0.5)
      ay+=0.001;
    printf("%.3f %.3f %.3f\n",ax,ay,r);
  }

}

  //y=a(x-a1)+b1
  //y=ax-a*a1+b1
  //ax-y=-a*a1+b1

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

using namespace std;

int main()
{
    int n;
    cin >> n;
    double x1,y1,x2,y2,x3,y3;
    while(n>0)
    {
        cin >> x1 >> y1 >>x2 >> y2 >> x3 >> y3;
        double s,v1x,v1y,v2x,v2y;
        v1x = y2-y1;
        v1y = x1-x2;

        v2x = y3-y1;
        v2y = x1-x3;
        double detA = v1x*v2y-v1y*v2x;
        double hx = (x2+x1)/2;
        double hy = (y2+y1)/2;
        s = (v2y*(x2-x1)/2 -v2x*(y2-y1)/2)/detA;
        double ox,oy;
        ox = hx + s*v1x;
        oy = hy + s*v1y;
        if(-0.0005<ox&&ox<0)ox = 0;
        if(-0.0005<oy&&oy<0)oy = 0;
        double r = sqrt((x1-ox)*(x1-ox)+(y1-oy)*(y1-oy));
        printf("%.3f %.3f %.3f\n",ox,oy,r);
        --n;
    }
    return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <cstdio>
#include <cmath>
#include <string>
#include <cctype>
#include <stack>
#include <queue>
#include <vector>
#include <cstdlib>
#include <list>
#define _USE_MATH_DEFINES
#include <math.h>

using namespace std;

int main() {

	int n;
	double x1, x2, x3, y1, y2, y3;
	double X, Y, R;
	double a, b, c, d, e, f;

	cin >> n;

	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		a = -x2 + x3;
		b = -x1*x1 + x2*x2 - y1*y1 + y2*y2;
		c = x1 - x2;
		d = -x2*x2 + x3*x3 - y2*y2 + y3*y3;
		e = -c * 2 * (y2 - y3);
		f = a * 2 * (y1 - y2);

		Y = (a*b + c*d) / (e - f);
		X = (b + 2 * Y*(y1 - y2)) / (-2 * c);
		R = sqrt((X - x1)*(X - x1) + (Y - y1)*(Y - y1));

		//?°???°????¬¬??????????????¨?????\
		if (X > 0)
			X = (int)(X * 1000 + 0.5);
		else X = (int)(X * 1000 - 0.5);

		if (Y>0)
			Y = (int)(Y * 1000 + 0.5);
		else Y = (int)(Y * 1000 - 0.5);

		if (R < 0)
			R = -R;
			R = (int)(R * 1000 + 0.5);

		printf("%.3f %.3f %.3f\n", X / 1000, Y / 1000, R / 1000);
	}

	return 0;
}