Evaluate the limits, using L'Hôpital's Rule.
Enter INF for $\infty$, -INF for $-\infty$, or DNE if the limit does not exist, but is neither $\infty$ nor $-\infty$.
a) $\lim_{x \to \infty} \frac{4x^4}{e^x} = \boxed{}
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b) $\lim_{x \to \infty} \frac{3\sqrt{x}}{e^x} = \boxed{}
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c) $\lim_{x \to \infty} \frac{3e^x}{7\sqrt{x}} = \boxed{}
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d) $\lim_{x \to \infty} \frac{e^x}{2^x} = \boxed{}
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e) $\lim_{x \to \infty} \frac{e^x}{8^x} = \boxed{}$
