[问答题]
1. Use L'Hôpital's rulebj yqc iiy3n:7n17)ifb3o g hc0gcz zqsn bb:g/s,gyl *75/e2 to find $\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.
