a) Ta có
$\lim \dfrac{2.3^{n+1} - 2^n}{5.4^n + 4} = \lim \dfrac{6. \left( \frac{3}{4} \right)^n - \left( \frac{1}{2} \right)^n}{5 + \frac{1}{4^{n-1}}}=0$
b) Ta có
$\lim \dfrac{6^{n-2} - 3.3^n}{4^n - 5} = \lim \dfrac{\frac{1}{36} - 3. \left( \frac{1}{2} \right)^n }{\left( \frac{2}{3} \right)^n - \frac{5}{6^n}} = 0$