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Is this series convergent?

Posted on 2011-10-10
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I'm doing a mock paper, with answers supplied. Here is one of the questions.

Is this series convergent?

SUM OF ((2n-1)/((4n^2)+1)
n=1 to infinity

This looks to me convergent, however...

the answer is that it is divergent by the Limit Comparison Test.

The details of the answer are:

let
an = ((2n-1)/((4n^2)+1)
and
bn = 1/n

then an>0, bn>0 and

(an/bn) = (2n^2-n)/(4n^2+1) = (2-1/n)/(4+1/n^2) -> 1/2 <> 0 as n->infinity

now
sum of bn from n=1 to infinity is divergent and so

sum of an is divergent from 1 to infinity by the Limit Comparison Test.

Have we not shown that an/bn tends to 1/2 as n->infinity, and hence that it is convergent ??
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Question by:purplesoup
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mwochnick earned 2000 total points
ID: 36942844
since an/bn -> 1/2 an and bn converge or diverge together - so since bn is divergent so is an
http://www.math.oregonstate.edu/home/programs/undergrad/CalculusQuestStudyGuides/SandS/SeriesTests/limit_comparison.html
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Author Comment

by:purplesoup
ID: 36943051
Oh yeh - I keep forgetting 1/n is divergent

http://en.wikipedia.org/wiki/Convergent_series

thanks for that.
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