# Help about ratio test for series..

• Dec 6th 2011, 12:50 PM
nappysnake
Help about ratio test for series..
I have a series and i have a problem doing the ratio test....

Σ k=1 to infinity, (-1)^k (1/2 + 1/k)^k

i face my problem with the 1/k :/

edit: (i have to actually examine the absolute convergence and i think i have to first examine the ratio..)
• Dec 6th 2011, 01:02 PM
mr fantastic
Re: Help about ratio test for series..
Quote:

Originally Posted by nappysnake
I have a series and i have a problem doing the ratio test....

Σ k=1 to infinity, (-1)^k (1/2 + 1/k)^k

i face my problem with the 1/k :/

Use the alternating series test (1/2 + 1/k)^k is monotone decreasing after a a certain value of n ....)
• Dec 6th 2011, 05:36 PM
Prove It
Re: Help about ratio test for series..
The ratio test will work too. By the ratio test, the series will be convergent if \displaystyle \begin{align*} \lim_{n \to \infty}\left|\frac{a_{n+1}}{a_n}\right| < 1 \end{align*}

\displaystyle \begin{align*} \lim_{n \to \infty}\left|\frac{a_{n+1}}{a_n}\right| &= \lim_{n \to \infty}\left|\frac{(-1)^{n+1}\left(\frac{1}{2} + \frac{1}{n+1}\right)^{n+1}}{(-1)^n\left(\frac{1}{2} + \frac{1}{n}\right)^n}\right| \\ &= \lim_{n \to \infty}\frac{\left(\frac{1}{2} + \frac{1}{n+1}\right)^{n+1}}{\left(\frac{1}{2} + \frac{1}{n}\right)^n} \\ &= \frac{1}{2} \textrm{ according to Wolfram Alpha.} \end{align*}

So the series converges.
• Dec 18th 2011, 01:21 AM
nappysnake
Re: Help about ratio test for series..
i have one further question. how would you calculate absolute convergence? wolfram alpha says that the ratio test is conclusive and that the series converges, but i have no idea how he comes to the result..all i get is a bunch of terms which i can't simplify..help?
• Dec 18th 2011, 02:31 AM
chisigma
Re: Help about ratio test for series..
Quote:

Originally Posted by nappysnake
I have a series and i have a problem doing the ratio test....

Σ k=1 to infinity, (-1)^k (1/2 + 1/k)^k

i face my problem with the 1/k :/

edit: (i have to actually examine the absolute convergence and i think i have to first examine the ratio..)

It is difficult to undestand why it is requested the ratio test instead of the root test, that extablishes that a series $\sum_{n=0}^{\infty} a_{n}$ converges if...

$\lim_{n \rightarrow \infty} \sqrt[n] {|a_{n}|} <1$ (1)

In that case is...

$\lim_{n \rightarrow \infty} \sqrt[n] {|a_{n}|} = \lim_{n \rightarrow \infty} (\frac{1}{2} + \frac{1}{n})= \frac{1}{2}$ (2)

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Marry Christmas from Serbia

$\chi$ $\sigma$