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 <iomanip>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <functional>

using namespace std;

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int,int> PII;
typedef long long ll;
typedef unsigned long long ull;

#define REP( i, m, n ) for ( int i = (int)( m ); i < (int)( n ); i++ )
#define UREP( i, m, n ) for ( unsigned int i = (unsigned int)( m ); i < (unsigned int)( n ); i++ )

#define ITER( c ) __typeof( (c).begin() )
#define IREP( c, it ) for ( ITER(c) it = c.begin(); it != c.end(); ++it )
#define ALL( c ) (c).begin(), (c).end()
#define RALL( c ) (c).rbegin(), (c).rend()

#define PB( n ) push_back( n )
#define MP( a, b ) make_pair( ( a ), ( b ) )
#define EXIST( c, e ) ( (c).find( e ) != (c).end() )

#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl
#define DEBUG( x ) cerr << __FILE__ << ":" << __LINE__ << ": " << #x << " = " << ( x ) << endl

double sqdist( double x1, double y1, double x2, double y2 )
{
	return pow( x1 - x2, 2 ) + pow( y1 - y2, 2 );
}

double dist( double x1, double y1, double x2, double y2 )
{
	return sqrt( sqdist( x1, y1, x2, y2 ) );
}

int main()
{
	int n;
	cin >> n;

	REP( dn, 0, n )
	{
		double x1, y1, x2, y2, x3, y3;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

		double mx = ( x1 + x2 ) / 2, my = ( y1 + y2 ) / 2;
		double a = -( ( x2 - x1 ) / ( y2 - y1 ) );
		double th = atan( a );

		double low = -100.0, high = 100.0, mid = 0;

		REP( i, 0, 1000 )
		{
			mid = ( low + high ) / 2;
			double cx = mx + cos( th ) * mid, cy = my + sin( th ) * mid;

			double d1 = sqdist( x1, y1, cx, cy ), d2 = sqdist( cx, cy, x3, y3 );

			if ( d1 < d2 )
			{
				high = mid;
			}
			else if ( d2 < d1 )
			{
				low = mid;
			}
		}

		double rx = mx + cos( th ) * mid, ry = my + sin( th ) * mid;

		cout.precision( 4 );
		cout << setiosflags( ios::showpoint ) << rx << ' ' << ry << ' ' << dist( x1, x2, rx, ry ) << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
#define x1 0
#define y1 1
#define x2 2
#define y2 3
#define x3 4
#define y3 5

int main()
{
	int n;
	double xy[6],a,b,c,d,e,f,x,y,r;

	cin>>n;
	for(int i=0;i<n;i++)
	{
		for(int j=0;j<6;j++)
			cin>>xy[j];

		a=2*(xy[x2]-xy[x1]);
		b=2*(xy[y2]-xy[y1]);
		c=2*(xy[x3]-xy[x2]);
		d=2*(xy[y3]-xy[y2]);
		e=xy[y2]*xy[y2]-xy[y1]*xy[y1]+xy[x2]*xy[x2]-xy[x1]*xy[x1];
		f=xy[y3]*xy[y3]-xy[y2]*xy[y2]+xy[x3]*xy[x3]-xy[x2]*xy[x2];

		x=(d*e-b*f)/(a*d-b*c);
		y=(a*f-c*e)/(a*d-b*c);
		r=sqrt((xy[x1]-x)*(xy[x1]-x)+(xy[y1]-y)*(xy[y1]-y));

		if(x<0)
			x-=0.0005;
		else x+=0.0005;
		if(y<0)
			y-=0.0005;
		else y+=0.0005;
		r+=0.0005;

		printf("%.3lf %.3lf %.3lf\n",x,y,r);
	}
}

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

using namespace std;

int main(void)
{
    int n;
    cin >> n;
    for(int i=0;i<n;i++){
        double x1,x2,x3;
        double y1,y2,y3;
        cin 
            >> x1 >> y1
            >> x2 >> y2
            >> x3 >> y3;
        double l12 = sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
        double l23 = sqrt((x2-x3)*(x2-x3)+(y2-y3)*(y2-y3));
        double l31 = sqrt((x3-x1)*(x3-x1)+(y3-y1)*(y3-y1));
        double cos123 = (l12*l12 + l23*l23 - l31*l31)/(2.0*l12*l23);
        double sin123 = sqrt(1.0-cos123*cos123);
        double r = l31 / (2.0*sin123);
        double coeff[4];
        double a = l23 * l23; // 1
        double b = l31 * l31; // 2
        double c = l12 * l12; // 3
        coeff[0] = a * ( b + c - a );
        coeff[1] = b * ( c + a - b );
        coeff[2] = c * ( a + b - c );
        coeff[3] = coeff[0] + coeff[1] + coeff[2];
        double xc = (coeff[0]*x1+coeff[1]*x2+coeff[2]*x3)/coeff[3];
        double yc = (coeff[0]*y1+coeff[1]*y2+coeff[2]*y3)/coeff[3];
#define sishagonyu(x) round(x*1000.0)/1000.0
        xc = sishagonyu(xc);
        yc = sishagonyu(yc);
        r  = sishagonyu(r);
        printf("%1.3f %1.3f %1.3f\n",
                xc,yc,r);
    }

    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <queue>
#include <algorithm>
#include <cmath>
#define INF 100000000
using namespace std;
#define pw(a) (a)*(a)

int main(){
  double x1,x2,x3,y1,y2,y3;
  int n;
  cin >> n;
  while(n--){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double a,b,l,m,n;
    a = -1*(pw(x1) - pw(x2) + pw(y1) - pw(y2));
    b = -1*(pw(x1) - pw(x3) + pw(y1) - pw(y3));
    l = (a*(y1-y3)-b*(y1-y2))/((y1-y3)*(x1-x2)-(x1-x3)*(y1-y2));
    m = ((x1-x3)*a - (x1-x2)*b)/((x1-x3)*(y1-y2)-(y1-y3)*(x1-x2));
    n = -1*(pw(x1) + pw(y1)+ l*x1 + m*y1);
    printf("%.3f %.3f %.3f\n",-l/2.0,-m/2.0,sqrt((pw(l) + pw(m) - 4*n))/2.0);
  }
  return 0;
}

C++: #include <bits/stdc++.h>  
 
using namespace std;
 
#define FOR(i, j, k) for(int i = j; i < k; ++i)
#define rep(i, j) FOR(i, 0, j)
#define repr(i, j) for(int i = j; i >= 0; --i)
#define INF (1 << 30)
 
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;

int main() {
	int N;
	scanf("%d", &N);
	rep(c, N) {
		double x1, y1, x2, y2, x3, y3;
		scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
		double p = ((y1-y3)*(y1*y1-y2*y2+x1*x1-x2*x2) - (y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3))/(2*(y1-y2)*(x1-x2)-2*(y1-y2)*(x1-x3));
		double q = ((x1-x3)*(x1*x1-x2*x2+y1*y1-y2*y2) - (x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3))/(2*(x1-x2)*(y1-y2)-2*(x1-x2)*(y1-y3));
		double r1 = sqrt( pow( p - x1, 2 ) + pow( q - y1, 2 ) );
        double r2 = sqrt( pow( p - x2, 2 ) + pow( q - y2, 2 ) );
        double r3 = sqrt( pow( p - x3, 2 ) + pow( q - y3, 2 ) );
        double r = ( r1 + r2 + r3 ) / 3.0;
		printf("%.3f %.3f %.3f\n", p, q, r);
	}
	return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main()
{
	int 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;
}

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

using namespace std;

int main()
{
    int n;
    cin >> n;
    double x1,y1,x2,y2,x3,y3;
    while(n>0)
    {
        cin >> x1 >> y1 >>x2 >> y2 >> x3 >> y3;
        double s,v1x,v1y,v2x,v2y;
        v1x = y2-y1;
        v1y = x1-x2;

        v2x = y3-y1;
        v2y = x1-x3;
        double detA = v1x*v2y-v1y*v2x;
        double hx = (x2+x1)/2;
        double hy = (y2+y1)/2;
        s = (v2y*(x2-x1)/2 -v2x*(y2-y1)/2)/detA;
        double ox,oy;
        ox = hx + s*v1x;
        oy = hy + s*v1y;
        //if(-0.0005<ox&&ox<0)ox = 0;
        //if(-0.0005<oy&&oy<0)oy = 0;
        double r = sqrt((x1-ox)*(x1-ox)+(y1-oy)*(y1-oy));
        printf("%.3f %.3f %.3f\n",ox,oy,r);
        --n;
    }
    return 0;
}