- #1
Joshk80k
- 17
- 0
Homework Statement
Show that
[tex]\sum \frac{n!}{10^n}[/tex]
converges or diverges.(Note, I was unsure of how to format this via latex, so the summation is from n = 1 to infinity.)
Homework Equations
The root test:
[tex]|\frac{a_n_+_1}{a_n}|[/tex]
The Attempt at a Solution
[tex] a_n=\frac{n!}{10^n},
a_n_+_1\frac{(n+1)!}{10^n^+^1} = \frac{(n+1)n!}{10^n^+^1}[/tex]
Applying the ratio test,
[tex]|\frac{a_n_+_1}{a_n}|= \frac{10^n(n+1)n!}{n!10^n^+^1} [/tex]
Cancelling terms out,
[tex]\frac{n+1}{10} = r[/tex]
Now, I know that if:
r > 1, it is divergent,
r < 1, convergent,
r = 1, inconclusive.
My problem is that I am not sure where to go after this. I still have an "n" in my answer, and I expected to just have a numerical answer.
I was going to just go ahead and say that since n approaches infinity, r is greater than 1, and thus the series is divergent, but I stopped because I realized that from n = 1 to n = 8, the series would be convergent, and worse still, at n = 9, the test would be inconclusive?
So my question is, did I make a mistake somewhere here, or is the ratio test not applicable here for this reason?
Thanks for any feedback!