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 <stdio.h>
#include <math.h>
using namespace std;

// _ÀWð^Æ·é
typedef complex<double> P;
// ÎÌè`
#define PI 3.1415926535898

// e·éë·Ã
#define EPS (1e-10)
//
#define X real()
#define Y imag()
	
// Op`NX
class Triangle{

private:
	//Op`ÌR_ÌÀW
	P a, b, c;
	//Op`ÌRÓÌ·³
	double edgeA,edgeB,edgeC;
	//Op`ÌRpÌå«³(WA)
	double angleA,angleB,angleC;
	//Op`ÌÊÏ
	double s;
	//]·è©çRÂÌpxðßéÖ
	double LawOfCosines(double a,double b, double c){
		return acos( (b*b+c*c-a*a) / (2.0*b*c) );
	}
	bool equal(double a, double b){//QÂÌdouble^Ìlªµ¢©Ç¤©
		return (fabs( a-b ) < EPS)? true : false ;
	}
	bool checkRightAngle(double a){//pª¼p©»è·é
		return ( fabs( a-(PI/2.0) ) < EPS )? true : false;
	}

public:
	//RXgN^(RÂÌ_ÆÓÆpxðú»)
	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 hoge;
		hoge = (edgeA+edgeB+edgeC)/2.0;
		s = sqrt(hoge*(hoge-edgeA)*(hoge-edgeB)*(hoge-edgeC));
	}
	
	
	double getAx(){//_AÌXÀWðÔ·
		return a.X;
	}
	double getAy(){//_AÌYÀWðÔ·
		return a.Y;
	}
	double getBx(){//_BÌXÀWðÔ·
		return b.X;
	}
	double getBy(){//_BÌYÀWðÔ·
		return b.Y;
	}
	double getCx(){//_CÌXÀWðÔ·
		return c.X;
	}
	double getCy(){//_CÌYÀWðÔ·
		return c.Y;
	}
	double getEdgeA(){//ÓAÌ·³ðÔ·
		return edgeA;
	}
	double getEdgeB(){//ÓBÌ·³ðÔ·
		return edgeB;
	}
	double getEdgeC(){//ÓCÌ·³ðÔ·
		return edgeC;
	}
	double getAngleA(){//pAÌpx(WA)ðÔ·
		return angleA;
	}
	double getAngleB(){//pBÌpx(WA)ðÔ·
		return angleB;
	}
	double getAngleC(){//pCÌpx(WA)ðÔ·
		return angleC;
	}
	double getArea(){//Op`ÌÊÏðÔ·
		return s;
	}
	double circumscribedCircleRadius(){//OÚ~Ì¼aðÔ·
		return ( edgeA / sin(angleA) / 2.0 );
	}
	double circumscribedCircleX(){//OS(OÚ~ÌS)ÌXÀWðÔ·
		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(){//OS(OÚ~ÌS)ÌYÀWðÔ·
		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) );
	}
	double incenterRadius(){//àÚ~Ì¼aðÔ·
		return ( (2.0*s) / (edgeA+edgeB+edgeC) );
	}
	double incenterX(){//àS(àÚ~ÌS)ÌXÀWðÔ·
		return ( (edgeA*a.X+edgeB*b.X+edgeC*c.X) / (edgeA+edgeB+edgeC) );
	}
	double incenterY(){//àS(àÚ~ÌS)ÌYÀWðÔ·
		return ( (edgeA*a.Y+edgeB*b.Y+edgeC*c.Y) / (edgeA+edgeB+edgeC) );
	}
	double centerOfGravityX(){//dSÌXÀWðÔ·
		return ( (a.X+b.X+c.X) / 3.0 );
	}
	double centerOfGravityY(){//dSÌYÀWðÔ·
		return ( (a.Y+b.Y+c.Y) / 3.0 );
	}
	bool checkRightTriangle(){//¼pOp`©Ç¤©»è·é
		if(checkRightAngle(angleA)||checkRightAngle(angleB)||checkRightAngle(angleC)){
			return true;
		}else{
			return false;
		}
	}
	bool checkIsoscelesTriangle(){//ñÓOp`©»è·é 
		if( equal(angleA,angleB) || equal(angleB,angleC) || equal(angleC,angleA) ){
			return true;
		}else{
			return false;
		}
	}
	bool checkEquilateralTriangle(){//³Op`©»è·é 
		if( equal(edgeA,edgeB) && equal(edgeB,edgeC) && equal(edgeC,edgeA)){
			return true;
		}else{
			return false;
		}
	}
};

int main(){
	int n;
	double ax,ay,bx,by,cx,cy;
	scanf("%d", &n);
	for(int i=0 ; i<n ; i++){
		scanf("%lf %lf %lf %lf %lf %lf", &ax,&ay,&bx,&by,&cx,&cy);
		P a(ax,ay), b(bx,by), c(cx,cy);
		Triangle Tr(a,b,c);
		printf("%.3f %.3f ", (float)Tr.circumscribedCircleX(), (float)Tr.circumscribedCircleY());
		printf("%.3f\n", (float)Tr.circumscribedCircleRadius() );
	}
	return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<math.h>
using namespace std;
int main(){
    int n;
    cin >> n;
    for(int i=0;i<n;i++){
        double x1,y1,x2,y2,x3,y3;
        cin >> x1 >>y1 >>x2 >> y2 >> x3 >> y3;
        double a1,a2,b1,b2,c1,c2,x,y,r;
         a1=2*(x2-x1);
         b1=2*(y2-y1);
         c1=x1*x1-x2*x2+y1*y1-y2*y2;
         a2=2*(x3-x2);
         b2=2*(y3-y2);
         c2=x2*x2-x3*x3+y2*y2-y3*y3;
         x=(b1*c2-b2*c1)/(a1*b2-a2*b1);
         y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
         r=hypot(x-x1,y-y1);
        printf("%.3lf %.3lf %.3lf\n",x,y,r);
    }
}

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

void solve(double x1, double x2, double x3, double y1, double y2, double y3) {
	double cx1 = (x1+x2)/2.0, cx2 = (x2+x3)/2.0;
	double cy1 = (y1+y2)/2.0, cy2 = (y2+y3)/2.0;
	double tx1 = y2-y1, tx2 = y3-y2;
	double ty1 = x2-x1, ty2 = x3-x2;
	double a=ty1, b=-tx1, c=cx1*ty1-cy1*tx1;
	double d=ty2, e=-tx2, f=cx2*ty2-cy2*tx2;
	double det = a*e-b*d;
	double x = (e*c-b*f)/det;
	double y = (-d*c+a*f)/det;
	double r = sqrt((x1-x)*(x1-x) + (y1-y)*(y1-y));
	printf("%.3f %.3f %.3f\n", x, y, r);
}

int main(void) {
	int n;
	scanf("%d", &n);
	for (int i = 0; i < n; i++) {
		double x1,x2,x3,y1,y2,y3;
		scanf("%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3);
		solve(x1, x2, x3, y1, y2, y3);
	}
	return 0;
}

C++: /*Filename: aoj0010.cpp
 *Author: Mushiyo
 */

#include <stdio.h>
#include <iostream>
#include <cmath>

using namespace std;

struct Point {
	double x;
	double y;
};

double countDist(Point a, Point b) {
	int xDist = abs(a.x - b.x);
	int yDist = abs(a.y - b.y);

	return sqrt(xDist * xDist + yDist * yDist);
}

Point getCircumCircleCenter(Point a, Point b, Point c) {
	Point bias = a;
	a.x = 0;
	a.y = 0;
	b.x -= bias.x;
	b.y -= bias.y;
	c.x -= bias.x;
	c.y -= bias.y;

	Point center;
	center.y = bias.y + ((c.x * c.x + c.y * c.y) * b.x - (b.x * b.x + b.y * b.y) * c.x)
			/ (2 * (b.x * c.y - c.x * b.y));
	center.x = bias.x + (-2 * center.y * b.y + b.x * b.x + b.y * b.y) / (2 * b.x);

	return center;
}

int main() {
	int n;

	while (cin >> n) {
		while (n > 0) {
			Point a, b, c;
			cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y;

			Point center = getCircumCircleCenter(a, b, c);
			double radius = countDist(center, a);

			printf("%.3f %.3f %.3f\n", center.x, center.y, radius);

			--n;
		}
	}

	return 0;
}