Integral test for converging series

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Homework Help Overview

The discussion revolves around the application of the integral test to determine the convergence or divergence of the series given by the summation from n=1 to infinity of 1/(n^2-4n-5). Participants are exploring the conditions under which the integral test is applicable, particularly focusing on the positivity, continuity, and monotonicity of the function involved.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants question how to verify if the function is positive, continuous, and decreasing, especially after encountering negative values for certain inputs. There is also a discussion about the necessity for the function to be monotone decreasing and non-negative across the entire summation interval for the integral test to be valid.

Discussion Status

The discussion is ongoing, with participants providing insights into the implications of negative function values and the behavior of specific terms in the series. Some express skepticism about the convergence of the series based on the undefined nature of certain terms, while others reference previous examples that suggest a different conclusion.

Contextual Notes

Participants note that the series diverges at n=5 due to the function approaching infinity, raising questions about the validity of applying the integral test in this scenario. There is also mention of conflicting results regarding the convergence of the integral itself.

lha08
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Homework Statement


In one of my problems, I'm asked to use integral test in order to determine whether the following function is converging or diverging:
(summation from n=1 to inf) 1/(n^2-4n-5)
I was just wondering how do we know if function is positive, continuous and decreasing because when i tried to see if it was positive (i plugged in a value), i got a negative...
Also, for a integral test, does the entire function have to be decreasing?

Homework Equations


The Attempt at a Solution

 
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lha08 said:

Homework Statement


In one of my problems, I'm asked to use integral test in order to determine whether the following function is converging or diverging:
(summation from n=1 to inf) 1/(n^2-4n-5)
I was just wondering how do we know if function is positive, continuous and decreasing because when i tried to see if it was positive (i plugged in a value), i got a negative...
Also, for a integral test, does the entire function have to be decreasing?

Is the summation really form n=1 to infinity? If so, I can tell you without testing that the series diverges (take a look at the the n=5 term:wink:)

Anyways, for the integral test to be a valid test, your summand (function) must be monotone decreasing and non-negative on the entire summation interval. If you plug in a value of 'n' (on the summation interval) and get a negative number you should know right there that the integral test is useless.
 


gabbagabbahey said:
Is the summation really form n=1 to infinity? If so, I can tell you without testing that the series diverges (take a look at the the n=5 term:wink:)

Anyways, for the integral test to be a valid test, your summand (function) must be monotone decreasing and non-negative on the entire summation interval. If you plug in a value of 'n' (on the summation interval) and get a negative number you should know right there that the integral test is useless.

You said that's it's apparently diverging, but in the example, we found it to be converging:(-1/6)ln(-2)...i don't know 'cause you mentioned that it has to be non-negative on its entire summation interval...:confused:
 


lha08 said:
You said that's it's apparently diverging, but in the example, we found it to be converging:(-1/6)ln(-2)...i don't know 'cause you mentioned that it has to be non-negative on its entire summation interval...:confused:

There is no way this series converges for n=1 to n=infinty... the problem is the n=5 term...

[tex]f(5)=1/(5^2-4(5)-5)=1/0=\infty[/tex]

How can a sum converge when one of its terms is undefined?!

Also, [tex]\int_1^\infty 1/(x^2-4x-5)dx\neq\frac{-1}{6}\ln(-2)[/tex] ...the integral doesn't even converge!
 

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