Explanation

(b)
$\lim_{n \rightarrow \infty}\frac{a^{n} + b^{n}}{a^{n} - b^{n}} = \lim_{n \rightarrow \infty}\frac{\left( \frac{a}{b} \right)^{n} + 1}{\left( \frac{a}{b} \right)^{n} - 1}$
$= - 1$
$\left\lbrack since,\ 0 < \frac{a}{b} < 1\ \ implies\left( \frac{a}{b} \right)^{n} \rightarrow 0\ as\ n \rightarrow \infty \right\rbrack$