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 "math.h"
#include "stdio.h"
#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, n;
	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);
		n = -(X1*X1+Y1*Y1+l*X1+m*Y1);
		printf("%.3f %.3f %.3f\n",l*-0.5,m*-0.5,sqrt(l*l+m*m-n*4.0)/2.0);
	}
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
double sisyagonyuu3(double moto){
	if(moto>0){
		return (moto+0.0005)/100*100;
	}else if(moto==0){
		return 0;
	}else{
		return (moto-0.0005)/100*100;
	}
}
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=sisyagonyuu3(((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=sisyagonyuu3(((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("%.3lf %.3lf %.3lf\n",p,q,r);
	}
	return 0;
}