Tính các giới hạn sau:
a) $\mathop {\lim }\limits_{n \to +\infty } (n^3+3n^2-1) b) \mathop {\lim }\limits_{n \to \infty }(2-
3n^3+2n^2) $
c) $\lim(\sqrt[]{3n^2+4}-\sqrt[]{2n^2+1} ) d) \lim(3^{n+2}-5^{n+1}+15)$
e) $\lim(3^n-\sin\sqrt[]{n}\frac{\pi}{2})$.
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