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<stdio.h>
#include<math.h>
using namespace std;

int main(){
	int num;
	cin>>num;
	for(int i=0;i<n;i++){
		double a,b,c,d,e,f;
		double x1,y1,x2,y2,x3,y3;
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
	
		a=x1-x2;
		b=y1-y2;
		c=x2-x3;
		d=y2-y3;
		e=(x2*x2+y2*y2)-(x1*x1+y1*y1);
		f=(x3*x3+y3*y3)-(x2*x2+y2*y2);
		double l=(e*d-f*b)/(a*d-c*b);
		double m=(c*e-a*f)/(c*b-a*d);
		double n=-1*x1*x1-(y1*y1)-l*x1-m*y1;
	
		double x=-1*l/2.0;
		double y=-1*m/2.0;
		double r=sqrt(l*l+m*m-4*n)/2.0;
		printf("%.3f %.3f %3f\n",x,y,r);
	}
}

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

using namespace std;

int main(){
  int n;
  cin>>n;
  for (int i=0;i<n;i++){
    double x1,x2,x3,y1,y2,y3;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    double a,b,c,d,e,f;
    a=-2*x1+2*x2;
    b=-2*y1+2*y2;
    c=x2*x2+y2*y2-x1*x1-y1*y1;
    d=-2*x1+2*x3;
    e=-2*y1+2*y3;
    f=x3*x3+y3*y3-x1*x1-y1*y1;
    double x=(c*d-a*f)/(b*d-a*e),y=(c*e-b*f)/(a*e-b*d);
    double r=(x1-x)*(x1-x)+(y1-y)*(y1-y);
    r=sqrt(r);
    if (x==0) x=0;
    if (y==0) y=0;
    if (r==0) r=0;
    printf("%.3f %.3f %.3f\n",round(x*1000)/1000.0,round(y*1000)/1000.0,round(r*1000)/1000.0);
  }
  return 0;
}

C++: #include <stdio.h>
#include <math.h>
int main(){
    double x[4],y[4];
    int n;
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        for(int i=0;i<3;i++)scanf("%lf%lf",&x[i],&y[i]);
        x[3]=y[0]*(x[2]*x[2]+y[2]*y[2])+y[1]*(x[0]*x[0]+y[0]*y[0])+y[2]*(x[1]*x[1]+y[1]*y[1])-y[0]*(x[1]*x[1]+y[1]*y[1])-y[1]*(x[2]*x[2]+y[2]*y[2])-y[2]*(x[0]*x[0]+y[0]*y[0]);
        y[3]=x[0]*(x[2]*x[2]+y[2]*y[2])+x[1]*(x[0]*x[0]+y[0]*y[0])+x[2]*(x[1]*x[1]+y[1]*y[1])-x[0]*(x[1]*x[1]+y[1]*y[1])-x[1]*(x[2]*x[2]+y[2]*y[2])-x[2]*(x[0]*x[0]+y[0]*y[0]);
        y[3]=-y[3];
        x[3]/=x[0]*y[1]+x[1]*y[2]+x[2]*y[0]-x[0]*y[2]-x[1]*y[0]-x[2]*y[1];
        y[3]/=x[0]*y[1]+x[1]*y[2]+x[2]*y[0]-x[0]*y[2]-x[1]*y[0]-x[2]*y[1];
        y[3]/=2;
        x[3]/=2;
        printf("%.3f %.3f %.3f\n",x[3],y[3],sqrt((x[3]-x[0])*(x[3]-x[0])+(y[3]-y[0])*(y[3]-y[0])));
    }
}

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

using namespace std;

int main()
{
	int n;
	double x1, y1, x2, y2, x3, y3;
	double xp, yp, r;
	double a1, b1, c1, a2, b2 ,c2;
	cin>>n;
	for(int i=0;i<n;++i)
	{
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		a1=2*(x2-x1);
		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;
		xp=(b1*c2-b2*c1)/(a1*b2-a2*b1);
		yp=(c1*a2-c2*a1)/(a1*b2-a2*b1);
		r=sqrt((x1+xp)*(x1+xp)+(y1+yp)*(y1+yp));
		printf("%.3lf %.3lf %.3lf\n", xp, yp, 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<stdio.h>
#include<math.h>

int main()
{
	int n;
	double x1[32],x2[32],x3[32],y1[32],y2[32],y3[32],p,q,r;
	double a,b,c;
	scanf("%d",&n);
	for(int i=0;i<n;i++){
		scanf("%lf%lf%lf%lf%lf%lf",&x1[i],&x2[i],&x3[i],&y1[i],&y2[i],&y3[i]);
	    p=(y1[i]-y3[i])*(y1[i]*y1[i]-y2[i]*y2[i]+x1[i]*x1[i]-x2[i]*x2[i])-(y1[i]-y2[i])*(y1[i]*y1[i]-y3[i]*y3[i]+x1[i]*x1[i]-x3[i]*x3[i]);
	    p/=(2*(y1[i]-y3[i])*(x1[i]-x2[i])-2*(y1[i]-y2[i])*(x1[i]-x3[i]));
	    q=(x1[i]-x3[i])*(x1[i]*x1[i]-x2[i]*x2[i]+y1[i]*y1[i]-y2[i]*y2[i])-(x1[i]-x2[i])*(x1[i]*x1[i]-x3[i]*x3[i]+y1[i]*y1[i]-y3[i]*y3[i]);
	    q/=(2*(x1[i]-x3[i])*(y1[i]-y2[i])-2*(x1[i]-x2[i])*(y1[i]-y3[i]));
		c=sqrt(fabs((x3[i]-x1[i])*(x3[i]-x1[i])+(y3[i]-y1[i])*(y3[i]-y1[i])));
		b=sqrt(fabs((x3[i]-x2[i])*(x3[i]-x2[i])+(y3[i]-y2[i])*(y3[i]-y2[i])));
		a=sqrt(fabs((x2[i]-x1[i])*(x2[i]-x1[i])+(y2[i]-y1[i])*(y2[i]-y1[i])));
		r=a*b*c/sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
		printf("%.3f %.3f %.3f\n",q,p,r);
	}
	return 0;
}
/*?????\???*/

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

using namespace std;

int main(void){
	int n;
	cin >> n;
	double x1,x2,x3,y1,y2,y3;
	for(int i = 0 ; i < n ; i ++){
		cin>>x1>>y1>>x2>>y2>>x3>>y3;
		double a = x2 * x2 - x1 * x1 + y2 * y2 - y1 * y1;
		double b = x3 * x3 - x1 * x1 + y3 * y3 - y1 * y1);
		
		double x = (2 * (y3 - y1) * a - 2 * (y2 - y1) * b ) / c;
		double y = (2 * (x3 - x1) * a - 2 * (x2 - x1) * b ) / c;
		double r = sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1));
		
		printf("%.3f %.3f %.3f\n",x,y,r);
		
	} 
	return 0;
}

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

/** Problem0010 : CIrcumscribed Circle of a Triangle **/
int main()
{
	int n;
	double X1, Y1, X2, Y2, X3, Y3;
	double a, b, c, d, e, f, l, m;
	cin >> n;
	for (int i=0; i<n; i++) {
		cin >> X1 >> Y1 >> X2 >> Y2 >> X3 >> Y3;
		a = X1-X2; b = Y1-Y2; c = -(X1*X1+Y1*Y1)+(X2*X2+Y2*Y2);
		d = X2-X3; e = Y2-Y3; f = -(X2*X2+Y2*Y2)+(X3*X3+Y3*Y3);
		l = (c*e - b*f)/(e*a - b*d); m = (c*d - a*f)/(b*d - a*e);
		cout << setprecision(3) << setiosflags(ios::fixed);
		cout << -l/2 << " " << -m/2 << " " << sqrt(l*l+m*m-4*n)/2 << endl;
	}
	return 0;
}