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Homework Statement
Show that the following series diverges
[tex]\sum_{n=1}^{\infty}\frac{n!}{2^{n}} [/tex]
Homework Equations
The Divergence Test: In order for a series to be divergent, the following must be true
[tex]\lim_{n\rightarrow \infty} a_n \neq 0 [/tex], or
[tex]\lim_{n\rightarrow \infty} a_n \nexists [/tex]
The Attempt at a Solution
Alright, I know how to work it out with the denominator, as it is a geometric series and therefore as [tex]n \rightarrow \infty,\ 2^{n} \rightarrow 1 [/tex]
But how do I do whenever I find a factorial? How do I work it out? I don't know what to do with this factorial, can I assume the following in this case
As [tex]n \rightarrow \infty[/tex],
[tex]n! \approx n[/tex]
Then as [tex]n \rightarrow \infty[/tex] it would summarize to
[tex]a_n = \frac{n}{2^{n}} [/tex], so by using L'hôpital's
[tex]\frac{\frac{d}{dn}n}{\frac{d}{dn} 2^{n}} [/tex]
[tex]\frac{1}{2^{n}}[/tex], and then as [tex]n \rightarrow \infty[/tex],
[tex]\frac{1}{1} = 1 \neq 0 [/tex]Is this it?
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