2.
$I=\lim\dfrac{(-1)^n\cos^n}{n^2+1}$
$\Big| \dfrac{(-1)^n\cos n}{n^2+1}\Big| \le \dfrac{1}{n^2+1}$
Vậy:
$I=\lim\dfrac{1}{n^2+1}=\lim\dfrac{ \dfrac{1}{n^2}}{1+\dfrac{1}{n^2}}=0$
3.
$L=\lim\dfrac{(-1)^n}{2^n+1}$
$\Big| \dfrac{(-1)^n}{2^n+1}\Big| \le \dfrac{1}{2^n+1}$
Vậy:
$I=\lim\dfrac{1}{2^n+1}=\lim\dfrac{ \dfrac{1}{2^n}}{1+\dfrac{1}{2^n}}=0$