Question 129
Find all real numbers $x$, $y$, $z$ which verify the equalities
$$
y = \frac{x^3 + 12x}{3x^2 + 4}, \quad z = \frac{y^3 + 12y}{3y^2 + 4}, \quad x = \frac{z^3 + 12z}{3z^2 + 4}.
$$
$$
y = \frac{x^3 + 12x}{3x^2 + 4}, \quad z = \frac{y^3 + 12y}{3y^2 + 4}, \quad x = \frac{z^3 + 12z}{3z^2 + 4}.
$$