[问答题]
1. Use L'Hôpital's ruzr,k7 wvy0 z/jba )*ompt18gb le to find kus8oj ifl5/o i/:)22 daxg pv $\lim _{x \rightarrow \infty} x^{3} e^{-x}$ .
2. Show that the proper integral $\int_{0}^{\infty} x^{3} e^{-x} \mathrm{~d} x$ converges, and state its value.
