Find the limits. \lim _{n \rightarrow \infty} n^{2}\left(\frac{1}{\sqrt{n^{2}+\cos n}}-\frac{1}{\sqrt{n^{2}+n}}\right) \lim _{n \rightarrow \infty}\left(\frac{8^{n}-3^{n} \cos \left(e^{n}\right)}{4^{n}+2^{n}}-2^{n}\right)