Đáp án:
20. $-4$
21.$-1$
Giải thích các bước giải:
20. $\lim\dfrac{1-4^{n+1}}{3^{n+1} + 4^n}$
$=\lim\dfrac{\left(\dfrac14\right)^n-4}{3\cdot\left(\dfrac34\right)^{n} + 1}$
$=\dfrac{0 - 4}{3.0 + 1}$
$= -4$
21. $\lim\dfrac{3^n - 6.4^n + 5^n}{3^n + 3.4^n - 5^n}$
$=\lim\dfrac{\left(\dfrac35\right)^n - 6\cdot\left(\dfrac45\right)^n +1}{\left(\dfrac35\right)^n + 3\cdot\left(\dfrac45\right)^n -1}$
$=\dfrac{0 - 6.0 + 1}{0 + 3.0 - 1}$
$= -1$
