Cho $\left ( X_{n} \right ),\left ( Y_{n} \right )$ xác định bởi: $\left\{\begin{matrix} X_{1}=-1 & & & & & \\ Y_{1}=1 & & & & & \\ X_{n+1}=-3X_{n}^{2}-2X_{n}.Y_{n}+8Y_{n}^{2} & & & & & \\ Y_{n+1}=2X_{n}^{2}+3X_{n}.Y_{n} -2Y_{n}^{2}& & & & & \end{matrix}\right.$
Đặt $U_{n}=\frac{X_{n}}{Y_{n}}$. Tính $X_{n},Y_{n},U_{n}$ theo n.