模板

#include<bits/stdc++.h>
using namespace std;

int n;
double r;

struct point {
    double x, y;
} p[100005], o;

inline double sqr(double x) { return x * x; }

inline double dis(point a, point b) {
    return sqrt(sqr(a.x - b.x) + sqr(a.y - b.y));
}

inline bool cmp(double a, double b) { return fabs(a - b) < 1e-8; }

point geto(point a, point b, point c) {
    double a1, a2, b1, b2, c1, c2;
    point ans;
    a1 = 2 * (b.x - a.x), b1 = 2 * (b.y - a.y),
    c1 = sqr(b.x) - sqr(a.x) + sqr(b.y) - sqr(a.y);
    a2 = 2 * (c.x - a.x), b2 = 2 * (c.y - a.y),
    c2 = sqr(c.x) - sqr(a.x) + sqr(c.y) - sqr(a.y);
    if(cmp(a1, 0)) {
        ans.y = c1 / b1;
        ans.x = (c2 - ans.y * b2) / a2;
    } else if(cmp(b1, 0)) {
        ans.x = c1 / a1;
        ans.y = (c2 - ans.x * a2) / b2;
    } else {
        ans.x = (c2 * b1 - c1 * b2) / (a2 * b1 - a1 * b2);
        ans.y = (c2 * a1 - c1 * a2) / (b2 * a1 - b1 * a2);
    }
    return ans;
}

int main() {
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)
        scanf("%lf%lf", &p[i].x, &p[i].y);
    for(int i = 1; i <= n; i++)
        swap(p[rand() % n + 1], p[rand() % n + 1]);
    o = p[1];
    for(int i = 1; i <= n; i++) {
        if(dis(o, p[i]) < r || cmp(dis(o, p[i]), r))
            continue;
        o.x = (p[i].x + p[1].x) / 2;
        o.y = (p[i].y + p[1].y) / 2;
        r = dis(p[i], p[1]) / 2;
        for(int j = 2; j < i; j++) {
            if(dis(o, p[j]) < r || cmp(dis(o, p[j]), r))
                continue;
            o.x = (p[i].x + p[j].x) / 2;
            o.y = (p[i].y + p[j].y) / 2;
            r = dis(p[i], p[j]) / 2;
            for(int k = 1; k < j; k++) {
                if(dis(o, p[k]) < r || cmp(dis(o, p[k]), r))
                    continue;
                o = geto(p[i], p[j], p[k]);
                r = dis(o, p[i]);
            }
        }
    }
    printf("%.10f
%.10f %.10f", r, o.x, o.y);
    return 0;
}