[问答题]
1. Use L'Hôpital's rulel;mm0 robf.6geoq t4u8 we 1z: to find w1d 6cdogii)8etkvsib9ba/13( -m qvl +j4ly $\lim _{x \rightarrow \infty} x^{3} e^{-x} $.
2. Show that the proper integral $\int_{0}^{\infty} x^{3} e^{-x} \mathrm{~d} x$ converges, and state its value.
