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 <math.h>
#include <algorithm>
#include <string>
#include <stack>
#include <queue>
#include <set>
#include <cstdio>
#include <string.h>
using namespace std;

#define FOR(i, n) for(int i = 0; i < n; i++)
#define bit(i) static_cast<bitset<8> >(i)

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> VI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long>PLL;
typedef queue<int> QI;
typedef priority_queue<int> maxpq;
typedef priority_queue<int, vector<int>, greater<int> > minpq;

struct edge{int to, cost;};

void begin(){cin.tie(0); ios::sync_with_stdio(false);};
int gcd(int a, int b){if(a%b==0){return(b);}else{return(gcd(b,a%b));}};
int lcm(int m, int n){if((0 == m)||(0 == n)){return 0;} return ((m / gcd(m, n)) * n);};
unsigned long long comb(long n, long m){unsigned long long c = 1; m = (n - m < m ? n - m : m);
    for(long ns = n - m + 1, ms = 1; ms <= m; ns ++, ms ++){c *= ns; c /= ms;} return c;};

void cp(int a1[], int a2[], int l){FOR(i, l) a2[i] = a1[i];};
double sq(double d){return d*d;};
int sq(int i){return i*i;};
double sqdist(int x1, int y1, int x2, int y2){
    double dx = x1 - x2, dy = y1 - y2; return dx*dx + dy*dy;
};

// ワーシャルフロイド法
void warshallFloyd(int d[100][100], int n){
    for(int k = 0; k < n; ++k)
        for(int i = 0; i < n; ++i)
            for(int j = 0; j < n; ++j)
                if(d[i][k] != -1 && d[k][j] != -1){
                    if(d[i][j] == -1){
                        d[i][j] = d[i][k] + d[k][j];
                    }else{
                        d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
                    }
                }
};


// d:隣接行列  n:グラフのサイズ  s:始点  dist:始点からの距離が入る配列
void dijkstra(int d[1000][1000], int n, int s, int dist[1000]){
    FOR(i, n) dist[i] = -1;
    dist[s] = 0;
    priority_queue<PII, vector<PII>, greater<PII> > q;
    q.push(PII(0, s));
    while (!q.empty()) {
        PII p = q.top(); q.pop();
        int i = p.second;
        if(dist[i] < p.first) continue;
        for(int j = 0; j < n; j++){
            if(d[i][j] == -1) continue;
            if(dist[j] == -1){
                dist[j] = dist[i] + d[i][j];
                q.push(PII(dist[j], j));
            }else if(dist[j] > dist[i] + d[i][j]){
                dist[j] = dist[i] + d[i][j];
                q.push(PII(dist[j], j));
            }
        }
    }
};

// 線分の交差判定
bool isCross(int x1, int y1, int x2, int y2, int x3, int y3, int x4, int y4){
    // 並行な場合
    int m = (x2-x1)*(y4-y3)-(y2-y1)*(x4-x3);
    if(m == 0) return false;
    // 媒介変数の値が0より大きく1以下なら交差する、これは問題の状況によって変わるかも。
    double r =(double)((y4-y3)*(x3-x1)-(x4-x3)*(y3-y1))/m;
    double s =(double)((y2-y1)*(x3-x1)-(x2-x1)*(y3-y1))/m;
    return (0 < r && r <= 1 && 0 < s && s <= 1);
};

// 外積の計算 AB CD の内積を求める
int crossProdct(int ax, int ay, int bx, int by, int cx, int cy, int dx, int dy){
    int abx = bx - ax; int aby = by - ay;
    int cdx = dx - cx; int cdy = dy - cy;
    return abx * cdy - cdx * aby;
};
double crossProdct(double ax, double ay, double bx, double by, double cx, double cy, double dx, double dy){
    double abx = bx - ax; double aby = by - ay;
    double cdx = dx - cx; double cdy = dy - cy;
    return abx * cdy - cdx * aby;
};
double innerProduct(double ax, double ay, double bx, double by, double cx, double cy, double dx, double dy){
    double abx = bx - ax; double aby = by - ay;
    double cdx = dx - cx; double cdy = dy - cy;
    return abx * cdx + aby * cdy;
};

// 三角形の内部判定 ABCの中にPがあるか判定
bool inTriangle(int ax, int ay, int bx, int by, int cx, int cy, int px, int py){
    int c1 = crossProdct(ax, ay, bx, by, bx, by, px, py);
    int c2 = crossProdct(bx, by, cx, cy, cx, cy, px, py);
    int c3 = crossProdct(cx, cy, ax, ay, ax, ay, px, py);
    return (c1 > 0 && c2 > 0 && c3 > 0) || (c1 < 0 && c2 < 0 && c3 < 0);
};
bool inTriangle(double ax, double ay, double bx, double by, double cx, double cy, double px, double py){
    double c1 = crossProdct(ax, ay, bx, by, bx, by, px, py);
    double c2 = crossProdct(bx, by, cx, cy, cx, cy, px, py);
    double c3 = crossProdct(cx, cy, ax, ay, ax, ay, px, py);
    return (c1 > 0 && c2 > 0 && c3 > 0) || (c1 < 0 && c2 < 0 && c3 < 0);
};

// 三角形の外心
void circumcenter(double ax, double ay, double bx, double by, double cx, double cy, double res[3]){
    // AB AC を求める
    double abx = bx - ax; double aby = by - ay;
    double acx = cx - ax; double acy = cy - ay;
    double m = abx * acy - acx * aby; // 分母 m = 0 となるのは3点が一直線になるとき
    double s = (abx * acx + aby * acy - sq(acx) - sq(acy)) / m; // 媒介変数s
    res[0] = abx / 2 + s * aby / 2; res[1] = aby / 2 - s * abx / 2;
    res[2] = sqrt(sqdist(0, 0, res[0], res[1]));
    res[0] += ax; res[1] += ay;
};




/**
 * start
 * @author yoshikyoto
 */
int main(int argc, const char * argv[]){
    begin();
    int n;
    cin >> n;
    FOR(i, n){
        double x1, y1, x2, y2, x3, y3;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        double res[3];
        circumcenter(x1, y1, x2, y2, x3, y3, res);
        printf("%.3lf %.3lf %.3lf\n", res[0], res[1], res[2]);
    }
}

C++: import java.util.Scanner;
import java.math.BigDecimal;

class Main{
public static void main(String[] args){
Scanner sc=new Scanner(System.in);
int n=sc.nextInt();
for(int i=0;i<n;i++){
double x1,y1,x2,y2,x3,y3;
x1=sc.nextDouble();
y1=sc.nextDouble();
x2=sc.nextDouble();
y2=sc.nextDouble();
x3=sc.nextDouble();
y3=sc.nextDouble();
double a=(x1*x1-x2*x2)*(y2-y3)-(x2*x2-x3*x3)-(y1-y2)*(y2-y3)*(y3-y1);
double b=2*((x1-x2)*(y2-y3)-(x2-x3)*(y1-y2));
double px=a/b;
double py;
if(y2!=y3){
py=(px-(x2+x3)/2)*(x3-x2)/(y2-y3)+(y2+y3)/2;
}
else{
py=(px-(x2+x1)/2)*(x1-x2)/(y2-y1)+(y2+y1)/2;
}
double r=Math.hypot(px-x1,py-y1);
BigDecimal x=new BigDecimal(px);
BigDecimal y=new BigDecimal(py);
BigDecimal z=new BigDecimal(r);
x=x.setScale(3,BigDecimal.ROUND_HALF_UP);
y=y.setScale(3,BigDecimal.ROUND_HALF_UP);
z=z.setScale(3,BigDecimal.ROUND_HALF_UP);
System.out.println(x.doubleValue()+" "+y.doubleValue()+" "+z.doubleValue());
}
}
}

C++: #include<bits/stdc++.h>
using namespace std;
#define mp make_pair
#define fi first
#define se second
#define ll long long
#define pb push_back

# define inOut(a,b) freopen(a,"r",stdin);freopen(b,"w",stdout);
# define RESET(a) memset(a,0,sizeof(a))
# define MEMO(a) memset(a,-1,sizeof(a))
# define DEBUG puts("Debug-Has-Come-Until-Here")


typedef pair<int,int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;

int main(){
	cin.tie(NULL);

	double x1, x2, x3, y1, y2, y3;
	double a,b,c,d,e,f;
	double x,y;
	int dummy;
	
	scanf("%d", &dummy);
	while(scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3)){
		a = -2*x1 + 2*x2;
		b = -2*y1 + 2*y2;
		c = (x2*x2 - x1*x1) + (y2*y2 - y1*y1);
		d = -2*x2 + 2*x3;
		e = -2*y2 + 2*y3;
		f = (x3*x3 - x2*x2) + (y3*y3-y2*y2);
		
		x = (c*e - b*f)/(a*e-b*d);
		y = (c*d - a*f)/(b*d-e*a);
		printf("%.3lf %.3lf %.3lf\n", x, y, sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1)));
	}

	return 0;
}