a(subscript)n = n!/n^n (hint: compare with 1/n)
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Note that for all integers n > 0
n!/n^n = (1 * 2 * 3 * ... * n) / (n * n * n * ... * n)
...........≤ (1 * n * n * ... * n) / (n * n * n * ... * n)
...........= n^(n-1) / n^n
...........= 1/n.
Hence, 0 ≤ n!/n^n ≤ 1/n.
Since lim(n→∞) 1/n = 0, we conclude by the Squeeze Theorem that lim(n→∞) n!/n^n = 0.
I hope this helps!
n!/n^n = (1 * 2 * 3 * ... * n) / (n * n * n * ... * n)
...........≤ (1 * n * n * ... * n) / (n * n * n * ... * n)
...........= n^(n-1) / n^n
...........= 1/n.
Hence, 0 ≤ n!/n^n ≤ 1/n.
Since lim(n→∞) 1/n = 0, we conclude by the Squeeze Theorem that lim(n→∞) n!/n^n = 0.
I hope this helps!