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## anonymous 5 years ago Sublimation n=1 to infinity of (cos(n•pi))/(srt(n)). How do I solve this using the alternating series test? Why is a sub n greater than or equal to zero. The answer is converges, but I need help walking thru the steps. Thanks.

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1. anonymous

You have a series,$\sum_{n=1}^{\infty}\frac{\cos n \pi}{\sqrt{n}}$It's important to realise that cos(n pi) will oscillate between -1 and 1 for each successive n. So your series is actually equal to$\sum_{n=1}^{\infty}\frac{(-1)^n}{\sqrt{n}}$

2. anonymous

The alternating series test says, basically, for a series of the form,$\sum_{n}(-1)^na_n$the series will converge if $\lim_{n \rightarrow \infty}a_n =0$and $a_{n+1}<a_n$for all n (i.e. if your terms go to zero as n goes to infinity, and where each new term is smaller than the last).

3. anonymous

Here, your a_n is$a_n=\frac{1}{\sqrt{n}}$which certainly goes to zero as n approaches infinity, and since sqrt(n) is monotonic increasing, it follows 1/sqrt(n) is monotonic decreasing, which is what you want.

4. anonymous

So, by the alternating series test, your series converges. Let me know if you need more help.

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