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 <cstdio>
#include <cmath>

using namespace std;

#define EPS 1e-9

struct P {
	double x, y;
};

double abs(P p) {
	return sqrt(p.x * p.x + p.y * p.y);
}
double dot(P p1, P p2) {
	return p1.x * p2.x + p1.y * p2.y;
}
double xx(double r) {
	return r*r;
}

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

		P a = (P){ x2 - x1, y2 - y1 };
		P b = (P){ x3 - x2, y3 - y2 };
		P c = (P){ x1 - x3, y1 - y3 };

		double R =  abs(a) / (sqrt(1 - xx(dot(b, c) / abs(b) / abs(c))) * 2);
		double l = sqrt( R*R - xx(abs(c)/2) );
		double gx = (x1+x3)/2, gy = (y1+y3)/2;
		P e = (P){ -c.y/abs(c), c.x/abs(c) };
		P eg = (P){ e.x * l, e.y * l };
		P g1 = (P){ gx + eg.x, gy + eg.y };
		P g2 = (P){ gx - eg.x, gy - eg.y };
		if ( abs(xx(x1-g1.x) + xx(y1-g1.y) - R) < EPS ) gx = g1.x, gy = g1.y;
		else gx = g2.x, gy = g2.y;
		printf("%.3f %.3f %.3f\n", gx, gy, R);
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
using namespace std;

#define FOR(i,a,b) for(long long int i=(a);i<(b);i++)
#define REP(i,n) for(long long int i=0;i<(n);i++)
#define ALL(s) (s).begin(),(s).end()

#define PI 3.14159265358479

typedef long long int ll;
typedef vector<long long int> vi;
typedef vector<long long int>::iterator vit;

void se(double a, double b, double c, double d, double e, double f,double *x,double *y){
	if (a < 0){
		a = -a;
		b = -b;
		c = -c;
	}
	if (b == -0)b = 0;
	if (d < 0){
		d = -d;
		e = -e;
		f = -f;
	}
	cout << a << " " << b << " " << c << " " << d << " " << e << " " << f << "\n";
	if (a == 0){
		*y = c / b;
		*x = (f - e*(*y)) / d;
	}
	else if (b == 0){
		*x = c / a;
		*y = (f - e*(*x)) / d;
	}
	if (d == 0){
		*y = f / e;
		*x = (c - b*(*y)) / a;
	}
	else if (e == 0){
		*x = f / d;
		*y = (c - a*(*x)) / b;
	}
	else if (a != 0 && b != 0 && d != 0 && e != 0){
		*x = (b*f - c*e) / (b*d - a*e);
		*y = (c*d - a*f) / (b*d - a*e);
	}
}


int main(){
	double x1, y1, x2, y2, x3, y3, px, py, r;
	ll n;
	cin >> n;
	REP(i,n){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		se(2.0*(x1 - x2), 2.0*(y1 - y2),
			pow(x1, 2.0) + pow(y1, 2.0) - pow(x2, 2.0) - pow(y2, 2.0),
			2.0*(x2 - x3), 2.0*(y2 - y3),
			pow(x2, 2.0) + pow(y2, 2.0) - pow(x3, 2.0) - pow(y3, 2.0), &px, &py);
		r = pow(pow(x1 - px, 2.0) + pow(y1 - py, 2.0), 0.5);

		printf("%.3lf %.3lf %.3lf", px, py, r);
	}
}