8. (Option 1) Find all solutions of the system $\left\{\begin{aligned} x y-t^{2} & =9 ; \\ x^{2}+y^{2}+z^{2} & =18\end{aligned}\right.$
Answer. $(3 ; 3 ; 0 ; 0),(-3 ;-3 ; 0 ; 0)$.

(Variant 2) Find all $x, y$ and $z$ satisfying the equations

$\int \log _{2}\left(x^{2}+1\right)+\log _{2}\left(y^{2}+1\right)=4$

$x^{2}+y^{2}=2 \cos ^{2} z+4$.

Answer. $( \pm \sqrt{3} ; \pm \sqrt{3} ; \pi n), n \in \mathbb{Z}$.