Understanding the nth Term Test for Divergence in Series

kamranonline
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I have a problem understanding the divergence of series.. There is a n-th term test that u first apply on the general term of the series and if its limit is not equal 0 then the series is divergent.. When i apply that test sometime i get it wrong and sometime not.. When can i apply this test? is there any special conditions for it? or it should always work.
 
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Please show us examples of what you mean by getting it wrong. The nth term test can tell you if a series diverges, but not all series that diverge fail the nth term test.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

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