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<math.h>
#include <iomanip>
using namespace std;
int main() {
	int n;
	cin >> n;
	float x1, y1, x2, y2, x3, y3;
	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

	float px, py, r;
	float A1, B1, C1, A2, B2, C2;
	A1 = 2 * (x2 - y2);
	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;
	
	px = (B1*C2 - B2*C1) / (A1*B2 - A2*B1);
	py = (C1*A2 - C2*A1) / (A1*B2 - A2*B1);

	r = sqrt((x1-px)*(x1-px)+(y1-py)*(y1-py));

	px = round(px * 1000) / 1000;
	py = round(py * 1000) / 1000;
	r = round(r * 1000) / 1000;
	cout <<fixed << setprecision(4) << px << " ";
	cout << fixed << setprecision(4) << py << " ";
	cout<< fixed << setprecision(4) << r << endl;

	return 0;

}

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

using namespace std;

int main(void){
	
	int n;
	double x[3]={0},y[3]={0},r,a,b,c,d[2]={0},e[2]={0},f[2]={0},l,m;

	cin>>n;

	for(int i=0;i<n;i++){
		
		a=b=c=r=l=m=0;
		for(int i=0;i<2;i++){
			d[i]=e[i]=f[i]=0;
		}


		cin>>x[0]>>y[0]>>x[1]>>y[1]>>x[2]>>y[2];

		a=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));	//ツ各ツ陛督づ個陳キツつウ
		b=sqrt((x[2]-x[1])*(x[2]-x[1])+(y[2]-y[1])*(y[2]-y[1]));
		c=sqrt((x[0]-x[2])*(x[0]-x[2])+(y[0]-y[2])*(y[0]-y[2]));	
	
		r=a*b*c/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));			//ツ外ツ静堋円ツづ個板シツ径
		
		d[0]=2*(x[1]-x[0]);
		e[0]=2*(y[1]-y[0]);
		f[0]=x[0]*x[0]-x[1]*x[1]+y[0]*y[0]-y[1]*y[1];
		d[1]=2*(x[2]-x[0]);
		e[1]=2*(y[2]-y[0]);
		f[1]=x[0]*x[0]-x[2]*x[2]+y[0]*y[0]-y[2]*y[2];

		l=(e[0]*f[1]-e[1]*f[0])/(d[0]*e[1]-d[1]*e[0]);
		m=(f[0]*d[1]-f[1]*d[0])/(d[0]*e[1]-d[1]*e[0]);

		l=l*1000;
		l=l+0.5;
		l=int(l);
		l=l/1000;

		m=m*1000;
		m=m+0.5;
		m=int(m);
		m=m/1000;

		r=r*1000;
		r=r+0.5;
		r=(int)(r);
		r=r/1000;
		
		printf("%0.3lf %0.3lf %0.3lf\n",l,m,r);
	}
	return 0;
}

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

double round3(double d)
{
    return floor(d * 1000.0 + 0.5) / 1000.0;
}

int main()
{
    int n;
    std::cin >> n;

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

        double p = ((x1*x1-x2*x2+y1*y1-y2*y2)*(y1-y3)-(x1*x1-x3*x3+y1*y1-y3*y3)*(y1-y2))/(2*(x1-x2)*(y1-y3)-2*(x1-x3)*(y1-y2));
        double q = ((x1*x1-x2*x2+y1*y1-y2*y2)*(x1-x3)-(x1*x1-x3*x3+y1*y1-y3*y3)*(x1-x2))/(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));
        double r = sqrt((x1 - p) * (x1 - p) + (y1 - q) * (y1 - q));

        std::cout << std::fixed << std::setprecision(3) << round3(p) << " " << round3(q) << " " << round3(r) << std::endl;
    }
}

C++: #include<cstdio>
#include<cmath>
 
//#include<iostream>
using namespace std;
 
int main(){
    int n;
    float x1, y1, x2, y2, x3, y3, xp,yp, xq, yq, s1, sr1, s2, sr2;
    float b1, b2, xc, yc, r;
    scanf("%d", &n);
    while(n--){
        scanf("%f%f%f%f%f%f",&x1, &y1, &x2, &y2, &x3, &y3);
 
        if(!(y2-y1)){ 
            r=x1; x1=x3; x3=r; r=y1; y1=y3; y3=r;
        }
        xp=(x2+x1)/2; yp=(y2+y1)/2;
        xq=(x3+x1)/2; yq=(y3+y1)/2;
        sr1=-(x2-x1)/(y2-y1); b1=yp-sr1*xp;
        sr2=-(x3-x1)/(y3-y1); b2=yq-sr2*xq;
 
        xc=-(b1-b2)/(sr1-sr2);
        yc=sr1*xc+b1;
        r=sqrt((xc-x1)*(xc-x1)+(yc-y1)*(yc-y1));
        printf("%.3f %.3f %.3f\n",xc, yc, r);
    }
    return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
using namespace std;
typedef vector <double> vd;
typedef vector < vd > vvd;

vvd inverse(vvd x){
	vvd tmp(2, vd(2));
	if (x.size()!=2) return tmp;
	double a, b, c, d, e;
	a = x[0][0];
	b = x[0][1];
    c = x[1][0];
	d = x[1][1];
    e = 1.0 / (a*d-b*c);
	tmp[0][0] = d * e;
	tmp[0][1] = -b * e;
	tmp[1][0] = -c * e;
	tmp[1][1] = a * e;
	return tmp;
}

vvd Mmult(vvd A, vvd B){
    int n1=A.size();
    int n2=A[0].size();
	int n3=B[0].size();
	vvd C(n1, vd(n3));
	for (int i=0; i<n1; i++)
		for (int j=0; j<n3; j++)
			C[i][j] = 0;

    for (int i=0; i<n1; i++)
		for (int j=0; j<n2; j++)
			for (int k=0; k<n3; k++)
				C[i][k] += A[i][j] * B[j][k];
    return C;
}

int main(void){
    int n;
	double x1, y1, x2, y2, x3, y3, dx1, dy1, dx2, dy2;
	vvd A(2, vd(2)), B(2, vd(1));
	cin>>n;
	for (int i=1; i<n+1; i++){
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		dx1 = x2 - x1;
		dy1 = y2 - y1;
		dx2 = x3 - x1;
		dy2 = y3 - y1;
		A[0][0] = -dx1;
		A[0][1] = dy1;
		A[1][0] = -dx2;
		A[1][1] = dy2;
		B[0][0] = -dx1*(x1+x2)/2 + dy1*(y1+y2)/2;
		B[1][0] = -dx2*(x1+x3)/2 + dy2*(y1+y3)/2;
		B = Mmult(inverse(A), B);

		double r = pow(pow(B[0][0]-x1,2)+pow(B[1][0]-y1,2), 0.5);
	    cout<<fixed<<setprecision(3);
		cout<<B[0][0]<<" "<<B[1][0]<<" "<<r<<endl;
	}
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int main() {
	int n;
	cin>>n;
	while(n--){
		double x1,x2,x3,y1,y2,y3;
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
	 	double a,b,c;
	 	a=pow((x2-x3),2)+pow((y2-y3),2);
	 	b=pow((x3-x1),2)+pow((y3-y1),2);
	 	c=pow((x1-x2),2)+pow((y1-y2),2);
	 	double ox,oy;
	 	ox=(a*(b+c-a)*x1+b*(c-b+a)*x2+c*(a+b-c)*x3)/(2*(a*b+b*c+c*a)-(a*a+b*b+c*c));
	 	oy=(a*(b+c-a)*y1+b*(c-b+a)*y2+c*(a+b-c)*y3)/(2*(a*b+b*c+c*a)-(a*a+b*b+c*c));
	 	double r;
	 	r=hypot((ox-x1),(oy-y1));
		printf("%.3lf %.3lf %.3lf\n",ox,oy,r);
	}
	return 0;
}

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

using namespace std;

double pow2(double x);

int main()
{
    int n;
    cin >> n;
    while (n--) {
        double x1, y1, x2, y2, x3, y3;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

        double xx1 = pow2(x1);
        double yy1 = pow2(y1);
        double xx2 = pow2(x2);
        double yy2 = pow2(y2);
        double xx3 = pow2(x3);
        double yy3 = pow2(y3);

        double a1 = 2 * (x2 - x1);
        double b1 = 2 * (y2 - y1);
        double c1 = xx1 - xx2 + yy1 - yy2;
        double a2 = 2 * (x3 - x1);
        double b2 = 2 * (y3 - y1);
        double c2 = xx1 - xx3 + yy1 - yy3;

        double xp = (b1 * c2 - b2 * c1) / (a1 * b2 - a2 * b1);
        double yp = (c1 * a2 - c2 * a1) / (a1 * b2 - a2 * b1);
        double r = sqrt(pow2(xp - x1) + pow2(yp - y1));

        cout << fixed << setprecision(3)
             << xp << " " << yp << " " << r << endl;
    }

    return 0;
}

double pow2(double x)
{
    return (x * x);
}