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

// ?????§?¨?????????¨??????
typedef complex<double> P;
// ???????????
#define PI 3.1415926535898

// ?¨±?????????????????
#define EPS (1e-10)
//XY??§?¨?
#define X real()
#define Y imag()

// ????§???¢?????????
class Triangle{

private:
	//????§???¢??????????????§?¨?
	P a, b, c;
	//????§???¢??????????????????
	double edgeA,edgeB,edgeC;
	//????§???¢???????§??????§??????(????????¢???)
	double angleA,angleB,angleC;
	//????????????????????????????§????????±?????????¢??°
	double LawOfCosines(double a,double b, double c){
		return acos( (b*b+c*c-a*a) / (2.0*b*c) );
	}


public:
	//?????????????????????(??????????????¨?????¨?§????????????????)
	Triangle(P p1, P p2, P p3){
		a = p1;
		b = p2;
		c = p3;
		edgeB = abs(c-a);
		edgeA = abs(b-c);
		edgeC = abs(a-b);
		angleA = LawOfCosines(edgeA,edgeB,edgeC);
		angleB = LawOfCosines(edgeB,edgeC,edgeA);
		angleC = LawOfCosines(edgeC,edgeA,edgeB);
	}

	double circumscribedCircleRadius(){//?????\?????????????????????
		return ( edgeA / sin(angleA) / 2.0 );
	}
	double circumscribedCircleX(){//??????(?????\????????????)???X??§?¨??????????
		double A = sin(2.0*angleA);
		double B = sin(2.0*angleB);
		double C = sin(2.0*angleC);
		return ( (a.X * A + b.X * B + c.X * C) / (A+B+C) );
	}
	double circumscribedCircleY(){//??????(?????\????????????)???Y??§?¨??????????
		double A = sin(2.0*angleA);
		double B = sin(2.0*angleB);
		double C = sin(2.0*angleC);
		return ( (a.Y * A + b.Y * B + c.Y * C) / (A+B+C) );
	}
};

int main(){
	int n;
	double x1, x2, x3, y1, y2, y3;
	double r, xp, yp;

	scanf("%d", &n);
	for (int i=0 ; i<n ; i++){
		scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
		P a( x1 , y1 );
		P b( x2 , y2 );
		P c( x3 , y3 );
		Triangle Tr(a,b,c);
		r  = Tr.circumscribedCircleRadius();
		xp = Tr.circumscribedCircleX();
		yp = Tr.circumscribedCircleY();
		printf("%.3f %.3f %.3f\n", xp,yp,r);
	}
}

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

inline double square(double x){return x*x;}
inline double Roundoff(double x){
	if( x<0 ){
		x*=-1;
		x=(x-(x-int(x*1000)/1000))+(((x*1000)-int(x*1000))*10)>=4?0.001:0;
		x*=-1;
	}else{
		x=(x-(x-int(x*1000)/1000))+(((x*1000)-int(x*1000))*10)>=4?0.001:0;
	}
	return x;
}

void SolveSimultEq(double &x,double &y,double a,double b,double c,double d,double e,double f);
int main(){
	int datanum;
	double *x1,*y1,*x2,*y2,*x3,*y3;
	cin>>datanum;
	x1=new double[datanum];
	y1=new double[datanum];
	x2=new double[datanum];
	y2=new double[datanum];
	x3=new double[datanum];
	y3=new double[datanum];
	
	for( int i=0;i<datanum;i++ ){
		cin>>x1[i]>>y1[i]>>x2[i]>>y2[i]>>x3[i]>>y3[i];
	}
	for( int i=0;i<datanum;i++ ){
		double px,py,r;
		
		SolveSimultEq(px,py,2*(x1[i]-x2[i]),2*(y1[i]-y2[i]),square(x1[i])-square(x2[i])+square(y1[i])-square(y2[i]),
  			  	    2*(x2[i]-x3[i]),2*(y2[i]-y3[i]),square(x2[i])-square(x3[i])+square(y2[i])-square(y3[i]));
		r=sqrt(square(x1[i]-px)+square(y1[i]-py));
		cout<<fixed<<setprecision(3)<<px<<" "<<setprecision(3)<<py<<" "<<setprecision(3)<<r<<endl;

	}

	delete x1;
	delete y1;
	delete x2;
	delete y2;
	delete x3;
	delete y3;
	return 0;
}

void SolveSimultEq(double &x,double &y,double a,double b,double c,double d,double e,double f){
	double delta=a*e-b*d;
	
	x=(c*e-b*f)/delta;
	y=(a*f-c*d)/delta;
}

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

int main() {
  int n;
  double x1, y1, x2, y2, x3, y3;
  double a1, b1, c1, a2, b2, c2;
  double x, y, r;
  cin >> n;
  while (n--) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    a1 = x2-x1;
    b1 = y2-y1;
    c1 = -a1*(x1+x2)*0.5 - b1*(y1+y2)*0.5;
    
    a2 = x3-x2;
    b2 = y3-y2;
    c2 = -a2*(x2+x3)*0.5 - b2*(y2+y3)*0.5;

    y = (a2*c1-a1*c2)/(a1*b2-a2*b1);
    if (a1) {
      x = -(b1*y+c1)/a1;
    } else {
      x = -(b2*y+c2)/a2;
    }
    r = sqrt((x1-x)*(x1-x) + (y1-y)*(y1-y));

    cout.setf(ios::fixed);
    cout.precision(3);
    cout << x << " " << y << " " << r << endl;
  }
}

C++: #include<iostream>
#include<cmath>
int main(){float a,b,c,d,e,f,g,h,i,j,k,l,n,s;for(std::cin>>n;n--;)std::cin>>g>>h>>i>>j>>k>>l,a=i-k,d=j-l,b=k-g,e=l-h,c=g-i,f=h-j,a=sqrt(d=d*d+a*a),b=sqrt(e=e*e+b*b),c=sqrt(f=f*f+c*c),s=(a+b+c)/2,s=sqrt(s*(s-a)*(s-b)*(s-c)),s*=s*16,a=d*(e+f-d),b=e*(f+d-e),c=f*(d+e-f),g-=d=(a*g+b*i+c*k)/s,h-=e=(a*h+b*j+c*l)/s,printf("%.3f %.3f %.3f\n",d,e,sqrt(g*g+h*h));}

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

struct TPoint
{
    double x;
    double y;
};
struct TTriangle
{
    TPoint p1;
    TPoint p2;
    TPoint p3;
};
void SolveEquation(double a, double b, double c, double d, double e, double f, double* x, double* y)
{
    *y = (f - d * c / a)/( e - d * b / a);
    *x = (c - b * *y) / a;
}
/*
A(a, b) B(c, d) C(e, f)
(OA)^2 = (OB)^2
(OA)^2 = (OC)^2
x2-2ax+a2 + y2-2by+b2 = x2-2cx+c2 + y2-2dy+d2
(2c-2a)x + (2d-2b)y = c^2 + d^2 - a^2 - b^2;

x2-2ax+a2 + y2-2by+b2 = x2-2ex+e2 + y2-2fy+f2
(2e-2a)x + (2f-2b)y = e^2 + f^2 - a^2 - d^2
*/
TPoint GetCentral(const TTriangle& Tri)
{
    using std::pow;
    double a, b, c, d, e, f;
    a = 2 * (Tri.p2.x - Tri.p1.x);
    b = 2 * (Tri.p2.y - Tri.p1.y);
    c = pow(Tri.p2.x,2) + pow(Tri.p2.y,2) - pow(Tri.p1.x,2) - pow(Tri.p1.y,2);
    d = 2 * (Tri.p3.x - Tri.p1.x);
    e = 2 * (Tri.p3.y - Tri.p1.y);
    f = pow(Tri.p3.x,2) + pow(Tri.p3.y,2) - pow(Tri.p1.x,2) - pow(Tri.p1.y,2);
    TPoint P;
    SolveEquation(a, b, c, d, e, f, &P.x, &P.y);
    return P;
}
double GetLength(const TPoint& A, const TPoint& B)
{
    // return std::sqrt(std::pow(A.x - B.x, 2) + std::pow(A.y - B.y, 2));
    return std::sqrt((A.x - B.x)*(A.x - B.x) + (A.y - B.y)*(A.y - B.y));
}
double GetRadius(double a, double b, double c)
{
    return (a*b*c)/std::sqrt((a+b+c)*(-a+b+c)*(a-b+c)*(a+b-c));
}
double GetRadius(const TTriangle& Tri)
{
    double a = GetLength(Tri.p1, Tri.p2);
    double b = GetLength(Tri.p2, Tri.p3);
    double c = GetLength(Tri.p3, Tri.p1);
    return GetRadius(a, b, c);
}
int main()
{
    int n;
    std::cin >> n;
    for(int i = 0; i < n; ++i){
        // double a, b, c;
        // std::cin >> a >> b >> c;
        // std::cout << GetRadius(a, b, c) << std::endl;
        
        TTriangle tri;
        std::cin >> tri.p1.x >> tri.p1.y >> tri.p2.x >> tri.p2.y >> tri.p3.x >> tri.p3.y;
        TPoint C = GetCentral(tri);
        double r = GetRadius(tri);
        std::cout << std::fixed << std::setprecision(3);
        if(C.x == -0) C.x = 0;
        if(C.y == -0) C.y = 0;
        if(r   == -0) r   = 0;
        std::cout << C.x << " " << C.y << " " << r << std::endl;
    }
    return 0;
}