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 <bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;

#define EPS (1e-10)
#define equals(a,b) (fabs((a)-(b))<EPS)
//CCW??¨///////////////////////////////////
static const int COUNTER_CLOCKWISE = 1;
static const int CLOCKWISE = -1;
static const int ONLINE_BACK = 2;
static const int ONLINE_FRONT = -2;
static const int ON_SEGMENT = 0;
/////////////////////////////////////////
class Point{
  public:
  double x,y;
  Point(double x=0,double y=0):x(x),y(y){}
  Point operator + (Point p){return Point(x+p.x,y+p.y);}
  Point operator - (Point p){return Point(x-p.x,y-p.y);}
  Point operator * (double a){return Point(a*x,a*y);}
  Point operator / (double a){return Point(x/a,y/a);}

  bool operator < (const Point &p) const{
    return x!=p.x?x<p.x:y<p.y;
  }
  bool operator == (const Point &p) const{
    return fabs(x-p.x)<EPS&&fabs(y-p.y)<EPS;
  }
};
struct Circle{
  Point c;
  double r;
};
typedef Point vect;
struct seg{Point p1,p2;};
//????????´?????????????????????
double norm(Point p){return p.x*p.x+p.y*p.y;}
//??¶????????????????????¬
double abs(Point p){return sqrt(norm(p));}
//?????????????????????????????????
double dot(Point a,Point b){
  return a.x*b.x+a.y*b.y;
}
//?????? ????????????????????????
double cross(Point a,Point b){
  return a.x*b.y-a.y*b.x;
}
//??´?????????????????¨?????\?????????????????????
bool C90(seg s1,seg s2){
  return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}
//????????????????????¨?????\??????????????????????????????????????????
bool C0(seg s1,seg s2){
  return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}
//?°???± ??????????????´???????????????
Point project(seg s,Point p){
  Point base = s.p2-s.p1;
  double r=dot(p-s.p1,base) / norm(base);
  return s.p1+base*r;
}
//????°????????????????????????????
Point reflection(seg s,Point p){
  return  p+(project(s,p)-p)*2.0;
}
//2???????????¢   ???????????????
double getDistancePP(Point a,Point b){
  return abs(a-b);
}
//??´??????????????¢(????°?)???????????????
double getDistanceLP(seg l,Point p){
  return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}
//?????????????????¢??????????????????
double getDistanceSP(seg s,Point p){
  if(dot(s.p2-s.p1,p-s.p1)<0.0)return abs(p-s.p1);
  if(dot(s.p1-s.p2,p-s.p2)<0.0)return abs(p-s.p2);
  return getDistanceLP(s,p);
}
//????¨?????????????????¨??????????????????????????????????
int CCW(Point p0,Point p1,Point p2){
  Point a=p1-p0;
  Point b=p2-p0;
  if(cross(a,b)>EPS)return COUNTER_CLOCKWISE;
  if(cross(a,b)<-EPS)return CLOCKWISE;
  if(dot(a,b)<-EPS)return ONLINE_BACK;
  if(norm(a)<norm(b))return ONLINE_FRONT;
  return ON_SEGMENT;
}
//????????????????????????????????????????????§?\????
bool intersect(Point p1,Point p2,Point p3,Point p4){
  return(CCW(p1,p2,p3)*CCW(p1,p2,p4)<=0&&CCW(p3,p4,p1)*CCW(p3,p4,p2)<=0);
}
bool intersect(seg s1,seg s2){
  return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}
//2??????????§???¢?????\??????????????????...????????????????????????
bool squareintersect(seg s1,seg s2){
  if(s1.p2.x<s2.p1.x||s2.p2.x<s1.p1.x)return 0;
  if(s1.p2.y<s2.p1.y||s2.p2.y<s1.p1.y)return 0;
  return 1;
}
//??????????????¢?????????????????????????????§??¨???????????????
double getDistance(seg s1,seg s2){
  if(intersect(s1,s2))return 0.0;
  return min(min(getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2)),
    min(getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)));
}
//??????????????????????????????(?´????????`)
Point getCrossPoint(seg s1,seg s2){
  Point base=s2.p2-s2.p1;
  double d1=abs(cross(base,s1.p1-s2.p1));
  double d2=abs(cross(base,s1.p2-s2.p1));
  double t=d1/(d1+d2);
  return s1.p1+(s1.p2-s1.p1)*t;
}
//?????¨??´???????????????2??????
seg getCrossPoint(Circle c,seg l){
  //assert(intersect(cc,l));
  Point pr=project(l,c.c);
  Point e=(l.p2-l.p1)/abs(l.p2-l.p1);
  double base =sqrt(c.r*c.r-norm(pr-c.c));
  seg pp;pp.p1=(pr+e*base);pp.p2=(pr-e*base);
  return pp;
}
//??????
Point gaishin(Point a, Point b, Point c){
  double a1,a2,b1,b2,c1,c2;
  a1=2*(b.x-a.x);b1=2*(b.y-a.y);c1=a.x*a.x-b.x*b.x+a.y*a.y-b.y*b.y;
  a2=2*(c.x-a.x);b2=2*(c.y-a.y);c2=a.x*a.x-c.x*c.x+a.y*a.y-c.y*c.y;
  Point p;p.x=(b1*c2-b2*c1)/(a1*b2-a2*b1);p.y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
  return p;
}

//????´???°2D///////////A???)///////////
typedef complex<double> comp;
typedef vector<comp> cvec;
//????´???????
double dot(comp a,comp b){return real(b*conj(a));}
//????´???????
double cross(comp a,comp b){return imag(b*conj(a));}
double Area(cvec &a){
  double b=0;int n=a.size();
  for(int i=0;i<n;i++)
    b+=cross(a[i],a[(i+1==n?0:i+1)]);
  return abs(b/2.0);
}
////how to use(exp)/////////////////////
//  double a,b; cvec t; cin>>n;       //
// while(n--){                        //
//    cin>>a>>b;                      //
//    t.push_back(comp(a,b));         //
//  }printf("%.11f\n",Area(t));       //
////////////////////////////////////////
//???????£????????????¢??????????????¢??????????????????????????\????????????????????§???????????£???????????????????????????????????§?????????????????????
int intersectCC(Circle a,Circle b){
  double dist=abs(a.c-b.c);
  if(dist>a.r+b.r+EPS)return 4;
  if(dist>a.r+b.r-EPS)return 3;
  if(dist>abs(a.r-b.r)+EPS)return 2;
  if(dist>abs(a.r-b.r)-EPS)return 1;
  return 0;
}

int main(){
int n;
cin>>n;
while(n--){
  Point a,b,c;
  cin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
  Point p=gaishin(a,b,c);
  printf("%.11f %.11f %.11f\n",p.x,p.y,getDistancePP(p,a));
}
}

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

struct Vector2D {
	double x, y;

	Vector2D() : x(0.0), y(0.0) {}
	Vector2D(double vx, double vy) : x(vx), y(vy) {}
	Vector2D& operator+=(const Vector2D& rhs)
	{
		x += rhs.x;
		y += rhs.y;
		return *this;
	}
	Vector2D& operator-=(const Vector2D& rhs)
	{
		x -= rhs.x;
		y -= rhs.y;
		return *this;
	}
	Vector2D& operator*=(double rhs)
	{
		x *= rhs;
		y *= rhs;
		return *this;
	}
};

Vector2D operator+(const Vector2D& lhs, const Vector2D& rhs)
{
	Vector2D result(lhs);
	return result += rhs;
}

Vector2D operator-(const Vector2D& lhs, const Vector2D& rhs)
{
	Vector2D result(lhs);
	return result -= rhs;
}

Vector2D operator*(double lhs, const Vector2D& rhs)
{
	Vector2D result(rhs);
	return result *= lhs;
}

void solve(Vector2D p1, Vector2D p2, Vector2D p3, Vector2D& center, double& radius)
{
	// p3-base p1, p3-base p2
	Vector2D r1(p1 - p3), r2(p2 - p3);

	double det = r2.x * r1.y - (-r1.x) * r2.y;
	double a = (r2.x * (r2.x - r1.x) + r2.y * (r1.y - r2.y)) * 0.5 / det;

	center = p3 + (0.5 * r1 + a * Vector2D(r1.y, -r1.x));

	Vector2D r = p1 - center;
	radius = std::sqrt(r.x * r.x + r.y * r.y);
}

int main()
{
	int count = 0;
	std::cin >> count;
	for (int i = 0; i < count; ++i) {
		Vector2D p1,p2,p3;
		std::cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y;
		if (!std::cin.good()) {
			break;
		}
		Vector2D center;
		double radius = 0.0;
		solve(p1, p2, p3, center, radius);
		std::printf("%.3f %.3f %.3f\n", center.x, center.y, radius);
	}

	return 0;
}