Zu (a)
$$ \lim_{k\to\infty} \frac{a_k}{a_{k+1}} = \lim_{k\to\infty} \frac{2^k}{2^{k+1}} = \lim_{k\to\infty} \frac{1}{2} = \frac{1}{2} $$
Zu (b)
$$ \lim_{k\to\infty} \frac{a_k}{a_{k+1}} = \lim_{k\to\infty} \frac{n^{k+1}(k+1)^2}{n^k k^2} = \lim_{k\to\infty} n \left( 1+\frac{1}{k} \right)^2 = n $$