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Test for Convergence

Date: 8/20/96 at 17:28:15
From: Anonymous
Subject: Test for Convergence

I cannot solve this problem:

Test the following series for convergence.

sum {from k=0 to infinity} [{log(k+1)-log k}/tan^(-1) (2/k)]

Tan^(-1) is tangent inverse. Because -1 < tan^(-1) < 1, I used the 
comparison test, but I did not achieve the goal.

Thanks for your kindness.

Date: 8/21/96 at 15:49:36
From: Doctor Anthony
Subject: Test for Convergence

This series appears to diverge since the term when k -> infinity is 
not zero.

When k->inf  term is ln(1+1/k)/tan^(-1)(2/k)

As this tends to 0/0 we can use l'Hopital's rule to find its limiting 

Differentiating top and bottom separately we get: 


 = (1/2){k/(k+1)}

 =  (1/2)k(k^2+4)

 = (1/2)(k^2+4)
      k^2 + k

 = (1/2)(1+4/k^2)
      1 + 1/k

If we let k->inf. This ratio -> 1/2

We can only get convergence if the terms tend to zero as k-> infinity, 
and this is not happening, so the series diverges.

-Doctor Anthony,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/   
Associated Topics:
High School Sequences, Series

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