(a) $\lim_{n \rightarrow \infty}\frac{2^{- n}(n^{2} + 5n + 6)}{(n + 4)(n + 5)} = \lim_{n \rightarrow \infty}\frac{n^{2}(1 + \frac{5}{n} + \frac{6}{n^{2}})}{2^{n}.n^{2}\left( 1 + \frac{4}{n} \right)(1 + \frac{5}{n})}$ $= 0$
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