a,
$\lim\Big(2^n+\dfrac{1}{n}\Big)$
$=\lim 2^n\Big(1+\dfrac{1}{n.2^n}\Big)$
$=+\infty$
b,
Ta có $n+3>0$.
$\lim\dfrac{2n^3+n-1}{n+3}$
$=\lim\dfrac{2+\dfrac{1}{n^2}-\dfrac{1}{n^3}}{\dfrac{1}{n^2}+\dfrac{3}{n^3}}$
$=+\infty$
c,
$\lim(5^n+(-2)^n)$
$=\lim 5^n\Big(1+\Big(\dfrac{-2}{5}\Big)^n \Big)$
$=+\infty$