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 <algorithm>
#include <functional>
#include <cmath>
#include <cstdio>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <bitset>
#include <climits>
#include <complex>
  
#define all(c) (c).begin(), (c).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define pb(e) push_back(e)
#define mp(a, b) make_pair(a, b)
#define fr first
#define sc second
  
const int INF=100000000;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
using namespace std;
typedef complex<double> P;
int n;
  
double cross(P p1,P p2) {
    return (real(p1)*imag(p2) - imag(p1)*real(p2));
}
double calc_S(P p1,P p2,P p3) {
    double ret = 0.0;
    p1 -= p3;
    p2 -= p3;
  
    ret = cross(p1,p2)/2.0;
      
    return abs(ret);
}
  
int main() {
    cin>>n;
    while(n-->0) {
        double x1_,y1_;
        double x2_,y2_;
        double x3_,y3_;
        cin>>x1_>>y1_>>x2_>>y2_>>x3_>>y3_;
        P p1(x1_,y1_);
        P p2(x2_,y2_);
        P p3(x3_,y3_);
        P A = p1-p2;
        P B = p2-p3;
        P C = p1-p3;
        double S = calc_S(p1,p2,p3);
        double R = abs(A)*abs(B)*abs(C)/S;
        R /= 4.0;
          
        P A1 = 2.0*(p2-p1);
        P B1 = 2.0*(p3-p1);
        P C1 = P(norm(p1)-norm(p2),norm(p1)-norm(p3));
        double X = cross(P(imag(A1),imag(B1)),C1)/cross(A1,B1);
        double Y = cross(C1,P(real(A1),real(B1)))/cross(A1,B1);
        printf("%.3lf %.3lf %.3lf\n",X,Y,R);
  
    }
    return 0;
}

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

using namespace std;

    // 半径取得の処理
double getR(double x1, double y1, double x2, double y2, double x3, double y3) {
    
    double marginX1, marginX2, marginX3;
    double marginY1, marginY2, marginY3;
    double a, b, c;
    double r, s;
    
        // それぞれのxとyの長さを求める
    marginX1 = abs(x2 - x1);
    marginY1 = abs(y2 - y1);
    marginX2 = abs(x1 - x3);
    marginY2 = abs(y1 - y3);
    marginX3 = abs(x3 - x2);
    marginY3 = abs(y3 - y2);
    
        // それぞれの辺の長さを求める
    a = sqrt(pow(marginX1,2) + pow(marginY1,2));
    b = sqrt(pow(marginX2,2) + pow(marginY2,2));
    c = sqrt(pow(marginX3,2) + pow(marginY3,2));
    
        // 半径を求める
    s = (a + b + c) / 2;
    r = (a * b * c) / (4 * sqrt(s * (s - a) * (s - b) * (s - c)));
    
    r *= 10000;
    r = (int)r;
    r /= 10;
    r = round(r);
    r /= 1000;
    
    return r;
}

    // 中心座標を求める処理
void getCentralCoodinate(double x1, double y1, double x2, double y2, double x3, double y3, double *xp, double *yp) {
    
    double marginX1, marginX2;
    double marginY1, marginY2;
    double marginXp1, marginXp2;
    double marginYp1, marginYp2;
    double n, m;
    
        // 初期化
    *xp = 0;
    *yp = 0;
    
        // それぞれのxとyの長さを求める
    marginX1 = abs(x2 - x1);
    marginY1 = abs(y2 - y1);
    marginX2 = abs(x1 - x3);
    marginY2 = abs(y1 - y3);
    
        // 中心座標を求めるのに必要な式
    marginXp1 = 2 * marginX1;
    marginYp1 = 2 * marginY1;
    marginXp2 = 2 * marginX2;
    marginYp2 = 2 * marginY2;
    
    n = pow(x1,2) - pow(x2,2) + pow(y1,2) - pow(y2,2);
    m = pow(x1,2) - pow(x3,2) + pow(y1,2) - pow(y3,2);
    
    *xp = (marginYp1 * m - marginYp2 * n) / (marginXp1 * marginYp2 - marginXp2 * marginYp1);
    *yp = (marginXp2 * n - marginXp1 * m) / (marginXp1 * marginYp2 - marginXp2 * marginYp1);
    
    *xp *= 10000;
    *xp = (int)*xp;
    *xp /= 10;
    *xp = round(*xp);
    *xp /= 1000;
    
    *yp *= 10000;
    *yp = (int)*yp;
    *yp /= 10;
    *yp = round(*yp);
    *yp /= 1000;
}

    // main処理
int main(void) {
    double x1, x2, x3;
    double y1, y2, y3;
    double xp, yp;
    double r;
    int n;
    
    cin >> n;
    
    while(cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
        
            // 半径と中心座標を取得する処理
        r = getR(x1, y1, x2, y2, x3, y3);
        getCentralCoodinate(x1, y1, x2, y2, x3, y3, &xp, &yp);
        
            // 出力
        cout << showpoint << setprecision(4);
        cout << xp << " " << yp << " " << r << endl;
    }
    
    return 0;
}

C++: #include <stdio.h>
#include <complex>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef complex<double> P;

double cross(const P& a, const P& b) { return imag(conj(a)*b); }
P rot90(const P& p) { return p*P(0, 1); }
P crosspoint(const P& l0, const P& l1, const P& m0, const P& m1) {
    const double A = cross(l1-l0, m1-m0);
    const double B = cross(l1-l0, l1-m0);
    return m0 + B/A * (m1-m0);
}

P center(P a, P b, P c) {
    const P m1 = 0.5*(a+b), m2 = 0.5*(b+c);
    return crosspoint(m1, m1+rot90(b-a), m2, m2+rot90(c-b));
}

int main() {
    int T;
    scanf("%d", &T);
    while (T--) {
        double x1, y1, x2, y2, x3, y3;
        scanf("%lf%lf", &x1, &y1);
        scanf("%lf%lf", &x2, &y2);
        scanf("%lf%lf", &x3, &y3);
        const P p = center(P(x1, y1), P(x2, y2), P(x3, y3));
        printf("%.3f %.3f %.3f\n", p.real(), p.imag(), abs(p-P(x1, y1)));
    }
    return 0;
}

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

int main(void)
{
	float x[3][2];
	int n;
	int xa, xb, xc;
	int ya, yb, yc;
	float ans[3];
	int i, j;
	
	scanf("%d", &n);
	
	while (n){
		for (i = 0; i < 3; i++){
			for (j = 0; j < 2; j++){
				scanf("%f", &x[i][j]);
			}
		}
		
		xa = 2 * (x[1][0] - x[0][0]);
		xb = 2 * (x[1][1] - x[0][1]);
		xc = x[0][0] * x[0][0] - x[1][0] * x[1][0] + x[0][1] * x[0][1] - x[1][1] * x[1][1];
		ya = 2 * (x[2][0] - x[0][0]);
		yb = 2 * (x[2][1] - x[0][1]);
		yc = x[0][0] * x[0][0] - x[2][0] * x[2][0] + x[0][1] * x[0][1] - x[2][1] * x[2][1];
		
		ans[0] = (xb * yc - yb * xc) / (xa * yb - ya * xb);
		ans[1] = (xc * ya - yc * xa) / (xa * yb - ya * xb);
		ans[2] = sqrt((ans[0] - x[0][0]) * (ans[0] - x[0][0]) + (ans[1] - x[0][1]) * (ans[1] - x[0][1]));
		
		for (i = 0; i < 3; i++){
			if (i){
				printf(" ");
			}
			printf("%.3f", ans[i]);
		}
		printf("\n");
		
		n--;
	}
	
	return (0);
}

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

int main() {
	int n;
	double x1, y1, x2, y2, x3, y3;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double x12, y12, x13, y13;
		x12 = (x1 + x2) / 2.0;
		y12 = (y1 + y2) / 2.0;
		x13 = (x1 + x3) / 2.0;
		y13 = (y1 + y3) / 2.0;
		double vx12, vy12, vx13, vy13;
		vx12 = x2-x1;
		vy12 = y2-y1;
		vx13 = x3-x1;
		vy13 = y3-y1;
		double x, y;
		x = (vx12*vy13*x12-vx13*vy12*x13+vy12*vy13*(y13-y12))/(vx12*vy13-vx13*vy12);
		y = y13 - vx13*(x-x13)/vy13;
		double r;
		r = sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1));
		printf("%.3f %.3f %.3f\n", x, y, r);
	}
	return 0;
}