What did i do wrong? Ratio and comparison test question?

In summary, the conversation is discussing the summation of n/lnn from n=1 to infinity and the use of the Ratio and Comparison test to determine its convergence. The person is questioning the validity of their approach using the Ratio test and asking for clarification on the logarithm rules. Another person suggests applying the divergence test to directly determine the limit.
  • #1
rickyram
2
0

Homework Statement



The summation of n/lnn from n=1 to infinity

Homework Equations



Ratio and Comparison test


The Attempt at a Solution



http://i55.tinypic.com/2jxuue.jpg

Alright so the way labeled "right way" on the picture is the right way according to the book but isn't the way I did with the ratio test right also? What did i do wrong?
 
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  • #2
It is NOT true that

[tex]\frac{ln(n+1)}{ln(n)}=ln(1) [/tex]
 
  • #3
ln (n+1) / ln (n) isn't ln(1), in the limit or otherwise.
 
  • #4
Oh man, I got to brush up on my logarithm rules... Anyway thanks for answerin my question, it has been a big help.
 
  • #5
Apply the divergence test directly. take limit n->(infinity) n/lnn. I doubt that goes to zero!
 

1. What is the ratio test and how does it work?

The ratio test is a method used to determine the convergence or divergence of a series. It involves taking the limit of the absolute value of the ratio of consecutive terms in the series. If the limit is less than 1, the series converges, and if it is greater than 1, the series diverges.

2. How do I know when to use the ratio test?

The ratio test is typically used when the terms in a series involve factorials, exponentials, or other complicated functions. It is also useful when dealing with series that involve both positive and negative terms.

3. Can the ratio test be used to determine the exact value of a series?

No, the ratio test only tells us whether a series converges or diverges. It does not provide the exact value of the sum.

4. Are there any limitations to the ratio test?

Yes, the ratio test can only be used for series with positive terms. It also may not be applicable for certain series that do not satisfy the necessary conditions for convergence or divergence.

5. How does the comparison test relate to the ratio test?

The comparison test is another method used to determine the convergence or divergence of a series. It involves comparing the given series to a known series with known convergence or divergence. The ratio test can be used to prove the convergence or divergence of the known series, which can then be used to determine the convergence or divergence of the given series.

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