Hey guys!

Final question....finally! (Whew) Can anyone help me with it please??

Investigate the convergence of the series

,

Thanks guys!

Jo (Flower)

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- May 21st 2008, 04:32 AMsimplysparklersSeries of positive and negative terms
Hey guys!

Final question....finally! (Whew) Can anyone help me with it please??

Investigate the convergence of the series

,

Thanks guys!

Jo (Flower) - May 21st 2008, 04:42 AMmr fantastic
- May 21st 2008, 04:43 AMPaulRS
Ok, note that is a decreasing sequence, and , thus by Leibniz Test converges

Alternating series test - Wikipedia, the free encyclopedia

You can do the same for the second one, or see that converges, thus must be convergent - May 21st 2008, 12:15 PMMathstud28
Just to clarify on PaulRS's completely correct statement

If converges this means that it converges absolutely

and if a series converges absolutely it converges conditionally(with the

But the converse is not necassarily true

I will use these two as examples

For the first one we first test for absolute convergence

So we check if converges

To this we notice that all the terms are positive and

So the integral test applies

So we set up the integral test

and so since the integral diverges the series diverges. So we know that this series is not absolutely convergent

but now we check for conditional convergence. To do this we use the alternating series test which states that if a series of the form and which means that if you can pick any number and after than number is monotonically decreasing ( )

If this applies then we can use the altnerating series test which states

that if the series is convergent

if the series is divergent

So since our series

Meets this criteria (e.g.

the alternating series test applies

So we check to see if

since direct substitution yields no indeterminate forms and its value is 0 we conclude the limit is 0

Therefore this series converges by the alternating series test

That showed that the converse of "if a series converges absolutely it converges conditionally" is untrue

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Now for your other series

we check for absolute convergence

so first we find

So either seeing that this is a convergent p-series or seeing that

so since the integral converges the series converges.

Now since we have shown that the series absolutely converges it MUST by definition converege conditionally

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Remember to follow instructions though. Generally when they ask you to find the convergence they mean conditional not absolute