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 <cmath>
#include <cstdio>
using namespace std;

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

        cin >> n;

        for(int i=0;i<n;i++){
                cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
                a = (x2-x1)*2.0;
                b = (y2-y1)*2.0;
                c = (x2*x2 + y2*y2 - x1*x1 - y1*y1);
                d = (x3-x1)*2.0;
                e = (y3-y1)*2.0;
                f = (x3*x3 + y3*y3 - x1*x1 - y1*y1);
                px = (c*e-b*f)/(a*e-b*d);
                py = (a*f-c*d)/(a*e-b*d);
                r = sqrt((px-x1)*(px-x1) + (py-y1)*(py-y1));
                printf("%.3f %.3f %.3f\n",px,py,r);



        }

        return 0;
}

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

using namespace std;

int main()
{
    int num;
    cin >> num;
    double x1, x2, x3, y1, y2, y3;
    double px, py, r;

    for(int i = 0; i < num; ++i)
    {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        px = (1 / 2.0) * ((x1 * x1 + y1 * y1) * (y3 - y2) + (x2 * x2 + y2 * y2) * (y1 - y3)
            + (x3 * x3 + y3 * y3) * (y2 - y1)) / (x1 * (y3 - y1) + x2 * (y1 - y3) + x3 * (y2 - y1));
        py = -(x3 - x2) / (y3 - y2) * px + (1 / 2.0) * (x3 * x3 - x2 * x2 + y3 * y3 - y2 * y2) / (y3 - y2);
        r = sqrt((x1 - px) * (x1 - px) + (y1 - py) * (y1 - py));

        cout << fixed << setprecision(3) << px << " " << py << " " << r << endl;
    }
}

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

int main() {
	int n;
	std::cin >> n;
	double rax, ray, rbx, rby, rcx, rcy;
	for (int i = 0; i < n; i++) {
		double ax, ay, bx, by, cx, cy, aa, bb, cc, a, b, c, d, e, f, rx, ry, rr;
		std::cin >> rax >> ray >> rbx >> rby >> rcx >> rcy;
		ax = rbx - rax; ay = rby - ray;
		bx = rcx - rbx; by = rcy - rby;
		cx = rax - rcx; cy = ray - rcy;
		aa = ax * ax + ay * ay;
		bb = bx * bx + by * by;
		cc = cx * cx + cy * cy;
		a = std::sqrt(aa);
		b = std::sqrt(bb);
		c = std::sqrt(cc);
		d = aa * (bb + cc - aa);
		e = bb * (cc + aa - bb);
		f = cc * (aa + bb - cc);
		rx = ((d * rcx) + (e * rax) + (f * rbx)) / (d + e + f);
		ry = ((d * rcy) + (e * ray) + (f * rby)) / (d + e + f);
		rr = (a * b * c) / std::sqrt((a + b + c)*(-a + b + c)*(a - b + c)*(a + b - c));
		std::cout << std::fixed;
		std::cout << std::setprecision(3) << rx << " " << ry << " " << rr << std::endl;
	}

	return 0;
}

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

int n;
double x[3],y[3];
struct data{
	double a,b,c;
	data(double aa,double bb,double cc){
		a=aa;
		b=bb;
		c=cc;
	}
};

struct data2{
	double x,y,r;
	data2(){

	}
	data2(double xx,double yy,double rr){
		x=xx;
		y=yy;
		r=rr;
	}
};

data make_line(int i,int j){
	if(x[i]==x[j]){
		return data(0.0,(y[i]+y[j])/2.0,1);
	}
	if(y[i]==y[j]){
		return data((x[i]+x[j])/2,0,0); 
	}
	double aa=(y[i]-y[j])/(x[i]-x[j]);
	aa=-1.0/aa;
	double bb=(y[i]+y[j])/2.0-(x[j]+x[i])/2.0*aa;
	return data(aa,bb,1);
}

data2 cross(data y1,data y2){
	data2 d;
	if(y1.c==0){
		d.x=y1.a;
		d.y=y2.a*d.x+y2.b;
	}
	if(y2.c==0){
		d.x=y2.a;
		d.y=y1.a*d.x+y1.b;
	}
	if(y1.c==1 && y2.c==1){
		d.x=(y1.b-y2.b)/(y1.a-y2.a);
		d.y=y1.a*d.x+y1.b;
	}
	d.r=sqrt(pow(d.x-x[0],2)+pow(d.y-y[0],2));
	return d; 
}

int main(void){
	scanf("%d",&n);
	for(int i=0;i<n;i++){
		for(int j=0;j<3;j++){
			scanf("%lf %lf",&x[j],&y[j]);
		}
		data y1=make_line(0,1);
		data y2=make_line(0,2);
		data2 y3=cross(y1,y2);
		printf("%.3f %.3f %.3f\n",y3.x,y3.y,y3.r);
	}
	return 0;
}

C++: #include <stdio.h>
#include <math.h>
double mx(double a, double b, double c) {if (a>=b && a>=c) return a; else if (b>=a && b>=c) return b; else return c;}
int main() {int N;while (scanf("%d",&N)!=EOF) {if (!N) return 0;while (N--) {double x1,y1,x2,y2,x3,y3,x,y;scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);x=mx(fabs(x1-x2),fabs(x1-x3),fabs(x2-x3))/2.00;y=mx(fabs(y1-y2),fabs(y1-y3),fabs(y2-y3))/2.00;printf("%.3lf %.3lf %.3lf\n",x,y,sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1)));}}}