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 <cmath>
using namespace std;

int main(){
	int n;
	cin >> n;
	double x[n], y[n], r[n];
	for(int i = 0; i < n; i++){
		double x1, y1, x2, y2, x3, y3;
		double a, b, c, d, e, f;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		a = 2*(x1-x2);
		b = 2*(y1-y2);
		c = 2*(x2-x3);
		d = 2*(y2-y3);
		e = x1*x1-x2*x2+y1*y1-y2*y2;
		f = x2*x2-x3*x3+y2*y2-y3*y3;
		x[i] = (d*e-b*f)/(a*d-c*b);
		y[i] = (a*f-c*e)/(a*d-b*c);
		r[i] = sqrt((x[i]-x1)*(x[i]-x1)+(y[i]-y1)*(y[i]-y1));
		if((int)floor(r[i]*10000)%10 > 4){
			r[i] = r[i] + 0.001;
		}
	}
	for(int i = 0; i < n; i++){
		cout <<  fixed << setprecision(3) << x[i] << " " << y[i] << " " << r[i] << endl;
	}
}

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

struct Zahyou
{
	float x, y;
};

int main()
{
	int n;
	Zahyou de[4];

	scanf("%d", &n);
	for (int i = 0; i < n; i++)
	{
		for (int j = 1; j < 4; j++)
		{
			scanf("%f %f", &de[j].x, &de[j].y);
		}
		de[0].x = ((de[1].y - de[3].y)*(pow(de[1].y, 2) - pow(de[2].y, 2) + pow(de[1].x, 2) - pow(de[2].x, 2)) + (de[1].y - de[2].y)*(pow(de[1].y, 2) - pow(de[3].y, 2) + pow(de[1].x, 2) - pow(de[3].x, 2))) / (2 * (de[1].y - de[3].y)*(de[1].x - de[2].x) + 2 * (de[1].y - de[2].y)*(de[1].x - de[3].x));
		de[0].y = ((de[1].x - de[3].x)*(pow(de[1].x, 2) - pow(de[2].x, 2) + pow(de[1].y, 2) - pow(de[2].y, 2)) + (de[1].x - de[2].y)*(pow(de[1].x, 2) - pow(de[3].x, 2) + pow(de[1].y, 2) - pow(de[3].y, 2))) / (2 * (de[1].x - de[3].x)*(de[1].y - de[2].y) + 2 * (de[1].x - de[2].x)*(de[1].y - de[3].y));
		
		printf("%.3f %.3f %.3f\n",de[0].x,de[0].y,sqrt(de[0].x+de[0].y));
	}
	return 0;
}

C++: #include<iostream>
#include<math.h>
#include<stdio.h>
using namespace std;
int main() {
    double x1, x2, x3, px, y1, y2, y3, py,r;
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        if (y1 == y2) {
            px = (x1 + x2) / 2;
            py = (x1 - x3)*(x2 - x3) / (2 * (y3 - y1)) + (y1 + y3) / 2;
        }
        else if (y1 == y3) {
            px = (x1 + x3) / 2;
            py = (x1 - x2)*(x3 - x2) / (2 * (y2 - y1)) + (y1 + y2) / 2;
        }
        else {
            px = ((x1*x1 - x2*x2) / (y2 - y1) - (x1*x1 - x3*x3) / (y3 - y1) + y3 - y2) / (2 * ((x1 - x2) / (y2 - y1) - (x1 - x3) / (y3 - y1)));
            py = (px - (x1 + x2) / 2)*(x1 - x2) / (y2 - y1) + (y1 + y2) / 2;
        }
        r = sqrt((px - x1)*(px - x1) + (py - y1)*(py - y1));
        printf("%.3f %.3f %.3f\n", px, py, r);
    }
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
int main(){
  double a[2],b[2],c[2];
  int n;
  cin >>n;
  while(n--){
    cin>>a[0]>>a[1]>>b[0]>>b[1]>>c[0]>>c[1];
    b[0]-=a[0];
    b[1]-=a[1];
    c[0]-=a[0];
    c[1]-=a[1];
    double m[2][2],ans[2],rev[2][2];
    m[0][0] = 2*b[0];
    m[0][1] = 2*b[1];
    ans[0] = b[0]*b[0]+b[1]*b[1];
    m[1][0] = 2*c[0];
    m[1][1] = 2*c[1];
    ans[1] = c[0]*c[0]+c[1]*c[1];
    double det = m[0][0]*m[1][1]-m[1][0]*m[0][1];
    rev[0][0] = m[1][1]/det;
    rev[1][0] = -m[1][0]/det;
    rev[0][1] = -m[0][1]/det;
    rev[1][1] = m[0][0]/det;
    double x = ans[0]*rev[0][0]+ans[1]*rev[0][1];
    double y = ans[0]*rev[1][0]+ans[1]*rev[1][1];
    double r = sqrt(x*x+y*y);
    printf("%.03lf %.03lf %.03lf\n",x+a[0],y+a[1],r);
  }
  return 0;
}

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

using namespace std;

int main()
{
	int i;
	cin >> i;
	while (i--)
	{
		double x1, x2, x3, y1, y2, y3;
		double X1, X2, Y1, Y2;
		double x0, y0, R1, R2, R3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		X1 = x2 - x1; X1 = X1 / 2;
		Y1 = y2 - y1; Y1 = Y1 / 2;
		X2 = x3 - x1; X2 = X2 / 2;
		Y2 = y3 - y1; Y2 = Y2 / 2;

		y0 = (X1*X2*X2 - X2*X1*X1 + X1*Y2*Y2 - X2*Y1*Y1) / (X1*Y2 - X2*Y1) + y1;
		x0 = (Y1*Y2*Y2 - Y2*Y1*Y1 + Y1*X2*X2 - Y2*X1*X1) / (Y1*X2 - Y2*X1) + x1;

		R1 = sqrt((x0 - x1)*(x0 - x1) + (y0 - y1)*(y0 - y1));

		printf("%.3lf %.3lf %.3lf", x0, y0, R1);
	}
	system("pause");
	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))));
		
#define AAA
#ifdef AAA
		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] << " " << q[i] << " " << r[i] << std::endl;
	}
#endif
#ifndef	AAA
	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] << " " << q[i] << " " << r[i] << std::endl;
	}
#endif
	//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<iostream>
#include<math.h>
#include<stdio.h>

int main(){
  int n;
  double a,b,c,d,e,f,x,y,r;
  double x1,y1,x2,y2,x3,y3;

  std::cin >> n;

  for(int i=0;i<n;i++){
    std::cin >> x1>>y1>>x2>>y2>>x3>>y3;
    a = 2*(x1-x2);
    b = 2*(y1-y2);
    c = +(x1*x1-x2*x2)+(y1*y1-y2*y2);
    d = 2*(x2-x3);
    e = 2*(y2-y3);
    f = +(x2*x2-x3*x3)+(y2*y2-y3*y3);

    x = (double)(c*e-b*f)/(a*e-b*d);
    y = (double)(a*f-c*d)/(a*e-b*d);

    if(x == 0){x = 0;}
    if(y == 0){y = 0;}

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

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