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++: /**************** Geometrical Library ****************/

#include<cmath>
#include<vector>
#include<algorithm>

#define	EPS	1e-8

enum {CCW=1,CW=-1,ON=0};

#define	mp	make_pair
#define	pb	push_back

using namespace std;

class Point{
public:
	double x,y;
	Point(){}
	Point(double xx,double yy):x(xx),y(yy){}
};

class Line:public vector<Point>{
public:
	Line(const Point &a,const Point &b){
		pb(a),pb(b);
	}
};

class Segment:public Line{
public:
	Segment(const Point &a,const Point &b):Line(a,b){}
};

class Polygon:public vector<Point>{};

class Circle{
public:
	Point c;
	double r;
	Circle(const Point &cc,double rr) : c(cc),r(rr){}
};

Point &operator +=(Point &a,const Point &b){
	a.x+=b.x,a.y+=b.y;
	return a;
}

Point &operator -=(Point &a,const Point &b){
	a.x-=b.x,a.y-=b.y;
	return a;
}

Point &operator *=(Point &a,double c){
	a.x*=c,a.y*=c;
	return a;
}

Point &operator /=(Point &a,double c){
	a.x/=c,a.y/=c;
	return a;
}

Point operator +(const Point &a,const Point &b){
	Point c=a;
	return c+=b;
}

Point operator -(const Point &a,const Point &b){
	Point c=a;
	return c-=b;
}

Point operator *(double c,const Point &a){
	Point b=a;
	return b*=c;
}

Point operator /(const Point &a,double c){
	Point b=a;
	return b/=c;
}

bool operator <(const Point &a,const Point &b){
	return (a.x==b.x)?(a.y<b.y):(a.x<b.x);
}

bool operator >(const Point &a,const Point &b){
	return b<a;
}

double dot(const Point &a,const Point &b){
	return a.x*b.x+a.y*b.y;
}

double cross(const Point &a,const Point &b){
	return a.x*b.y-a.y*b.x;
}

double dis2(const Point &a,const Point &b){
	return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}

int ccw(const Point &a,Point b,Point c){
	b-=a,c-=a;
	double rotdir=cross(b,c);
	if(rotdir>EPS)		return CCW;
	if(rotdir<-EPS)		return CW;
	return ON;
}

Polygon ConvexHull(vector<Point> ps){
	int n=ps.size();
	sort(ps.begin(),ps.end());

	vector<Point> ch_u(n);
	int n_u=0;
	for(int i=n-1;i>=0;i--){
		ch_u[n_u++]=ps[i];
		while(n_u>=3 && ccw(ch_u[n_u-3],ch_u[n_u-2],ch_u[n_u-1])!=CCW){
			ch_u[n_u-2]=ch_u[n_u-1];
			n_u--;
		}
	}

	vector<Point> ch_l(n);
	int n_l=0;
	for(int i=0;i<n;i++){
		ch_l[n_l++]=ps[i];
		while(n_l>=3 && ccw(ch_l[n_l-3],ch_l[n_l-2],ch_l[n_l-1])!=CCW){
			ch_l[n_l-2]=ch_l[n_l-1];
			n_l--;
		}
	}

	Polygon ch;
	for(int i=0;i<n_u-1;i++)	ch.pb(ch_u[i]);
	for(int i=0;i<n_l-1;i++)	ch.pb(ch_l[i]);
	return ch;
}

bool parallel(const Line &l,const Line &m){
	return abs(cross(l[1]-l[0],m[1]-m[0]))<EPS;
}

bool orthogonal(const Line &l,const Line &m){
	return abs(dot(l[1]-l[0],m[1]-m[0]))<EPS;
}

inline void calc_abc(const Line &l,double &a,double &b,double &c){	// l : ax+by+c=0
	a=l[0].y-l[1].y;
	b=l[1].x-l[0].x;
	c=l[0].x*l[1].y-l[1].x*l[0].y;
}

bool intersect(const Line &l,const Line &m,Point *p=NULL){
	// this routine also returns true in case "M is on L", "M sessuru L", etc,.
	if(abs(cross(l[1]-l[0],m[1]-m[0]))>EPS
	|| abs(cross(l[1]-l[0],m[0]-l[0]))<EPS){
		if(p){
			double a1,b1,c1,a2,b2,c2;
			calc_abc(l,a1,b1,c1);
			calc_abc(m,a2,b2,c2);
			double det=a1*b2-a2*b1;
			if(abs(det)<EPS)	*p=l[0];	// l == m
			else{
				p->x=(b1*c2-b2*c1)/det;
				p->y=(a2*c1-a1*c2)/det;
			}
		}
		return true;
	}
	return false;
}

bool intersect(const Segment &s,const Segment &t,Point *p=NULL){
	if(max(s[0].x,s[1].x)<min(t[0].x,t[1].x)
	|| max(t[0].x,t[1].x)<min(s[0].x,s[1].x)
	|| max(s[0].y,s[1].y)<min(t[0].y,t[1].y)
	|| max(t[0].y,t[1].y)<min(s[0].y,s[1].y))	return false;

	if(ccw(s[0],s[1],t[0])*ccw(s[0],s[1],t[1])<=0
	&& ccw(t[0],t[1],s[0])*ccw(t[0],t[1],s[1])<=0){
		if(p){
			double a1,b1,c1,a2,b2,c2;
			calc_abc(s,a1,b1,c1);
			calc_abc(t,a2,b2,c2);
			double det=a1*b2-a2*b1;
			if(abs(det)<EPS){	// s is parallel to t
				Point q[3]={s[0],s[1],t[0]};
				for(int i=0;i<3;i++){
					if(dot(q[i]-s[0],q[i]-s[1])<EPS && dot(q[i]-t[0],q[i]-t[1])<EPS){
						*p=q[i];
						break;
					}
				}
			}
			else{
				p->x=(b1*c2-b2*c1)/det;
				p->y=(a2*c1-a1*c2)/det;
			}
		}
		return true;
	}
	return false;
}

Point perp_foot(const Point &p,const Line &l){
	double a,b,c;
	calc_abc(l,a,b,c);
	return p-(a*p.x+b*p.y+c)/(a*a+b*b)*Point(a,b);
}

Line perp_bisector(const Point &a,const Point &b){
	return Line(Point((a.x+a.y+b.x-b.y)/2,(b.x+b.y-a.x+a.y)/2),
				Point((b.x+b.y+a.x-a.y)/2,(a.x+a.y-b.x+b.y)/2));
}

double area(const Point &a,const Point &b,const Point &c){
	return abs(cross(b-a,c-a))/2;
}

double area(Polygon &pl){	// !! only convex polygon
	int n=pl.size();
	double a=0;
	for(int i=0;i<n-2;i++)	a+=area(pl[0],pl[i+1],pl[i+2]);
	return a;
}

/**************** Library END ****************/

#include<cstdio>

int main(){
	int n;	scanf("%d",&n);
	while(n--){
		Point a,b,c;	scanf("%lf%lf%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y,&c.x,&c.y);
		Line l=perp_bisector(a,b),m=perp_bisector(b,c);
		Point p;
		intersect(l,m,&p);
		double r=sqrt(dis2(p,a));
		printf("%.3f %.3f %.3f\n",p.x,p.y,r);
	}
	return 0;
}

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

double sisya(double x)
{
	int a = static_cast<int>(x * 10000);
	x = static_cast<double>(static_cast<int>(x * 1000)) / 1000;
	if(a < 0 && a%10 <= -5)
		return x-0.001;
	else if(a%10 >= 5)
		return x+0.001;
	return x;
}

double dif(double x,double y,double xx, double yy)
{
	return sqrt((x-xx)*(x-xx)+(y-yy)*(y-yy));
}

int main()
{
	int n;
	double x1,x2,x3,y1,y2,y3;
	cin >> n;
	for(int i=0;i<n;i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
		double maxx = max(x1,max(x2,x3));
		double maxy = max(y1,max(y2,y3));
		double minx = min(x1,min(x2,x3));
		double miny = min(y1,min(y2,y3));
		cout << setprecision(3) << fixed << sisya(maxx - (maxx - minx) / 2 )  << " ";
		cout << setprecision(3) << fixed << sisya(maxy - (maxy - miny) / 2 )  << " ";
		cout << setprecision(3) << fixed << sisya(dif(x1,y1,maxx - (maxx - minx) / 2, maxy - (maxy - miny) / 2))<< endl;
	}
	return 0;
}

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

int main()
{
	int n;
	double x1,x2,x3,y1,y2,y3,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,&x2,&x3,&y1,&y2,&y3);
	    p=(y1-y3)*(y1*y1-y2*y2+x1*x1-x2*x2)-(y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3);
	    p/=(2*(y1-y3)*(x1-x2)-2*(y1-y2)*(x1-x3));
	    q=(x1-x3)*(x1*x1-x2*x2+y1*y1-y2*y2)-(x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3);
	    q/=(2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3));
		c=sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
		b=sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2));
		a=sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
		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 <iomanip>
#include <cmath>
using namespace std;
int main()
{
	double n;
	double x1, x2, x3;
	double y1, y2, y3;
	cin >> n;
	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
	double px, py, r;
	double A = (x2 - x1)*(y3 - y2) - (y2 - y1)*(x3 - x2);
	px = 0.5*((y3 - y2)*(x2*x2 - x1*x1 + y2*y2 - y1*y1) - (y2 - y1)*(x3*x3 - x2*x2 + y3*y3 - y2*y2)) / A;
	py = 0.5*(-(x3 - x2)*(x2*x2 - x1*x1 + y2*y2 - y1*y1) + (x2 - x1)*(x3*x3 - x2*x2 + y3*y3 - y2*y2)) / A;
	double a = px - x1;
	double b = py - y1;
	double r0 = (a*a + b*b);
	double r1 = sqrt(r0);
	cout << fixed << setprecision(10) << px << ' ' << py << ' ' << r1 <<  endl;
}