Evaluate the limits, using L'Hôpital's Rule where appropriate.
Enter INF for $\infty$, -INF for $-\infty$, or DNE if the limit does not exist, but is neither $\infty$ nor $-\infty$.

a) $\displaystyle \lim_{x\to \infty} (4 x)^{10 / x} =$

b) $\displaystyle \lim_{x\to \infty} (4 / x)^{13 x} =$