MacLaurin Series: Showing 1/n(n+1) = 1

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Homework Help Overview

The discussion revolves around using the MacLaurin series for e^x and ln(1+x) to demonstrate the equality of an infinite series involving the terms 1/n(n+1). Participants are exploring the connections between series expansions and the problem statement.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Some participants question the necessity of using the MacLaurin series for e^x and ln(1+x), suggesting alternative methods such as simplifying the series using partial fractions.
  • Others express confusion about how to apply the series expansions effectively, noting that there are multiple related questions on the homework sheet that may require similar techniques.
  • There are suggestions to integrate the logarithmic function after multiplying it by x, with participants discussing the outcomes of their attempts.
  • Some participants reflect on the challenges of integrating correctly and the implications of their results on the original problem.

Discussion Status

The discussion is active, with participants sharing various approaches and insights. While some have made progress on related problems, there is no explicit consensus on the best method to tackle the original question. Guidance has been offered, particularly around integration and series manipulation, but uncertainty remains regarding the application of these techniques to the initial problem.

Contextual Notes

Participants note that there are multiple questions on the homework sheet, which may influence their approach to the current problem. There is also mention of potential confusion arising from the requirement to use specific series expansions.

EEWannabe
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Homework Statement


Use the MacLaurin series for e^x and ln (1+x) to show that;

\frac{1}{1*2}+\frac{1}{2*3}+\frac{1}{3*4}...= 1

Homework Equations



e^{x}= 1 + x + \frac{x^{2}}{2!}+\frac{x^{3}}{3!}...

ln(1+x)= x - \frac{x^{2}}{2}+\frac{x^{3}}{3}...


The Attempt at a Solution



Well I'm not really sure how to go about this really, any sort of guidance would be great. Obviously it's in the form 1/n(n+1) but I'm not sure really :<
 
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Not sure why you need the MacLaurin series of e^x and ln...

Observe that \frac{1}{n(n+1)}=\frac{1}{n}-\frac{1}{n+1}. You can simplify the series with that...
 
Hmmm that's where I'm getting confused.

I can see that you can expand as you said and use the method of differences to show that it's 1 but it says to use the expansions.

Also there are about 5/6 other questions on the sheet including

<br /> \frac{1}{1*2}-\frac{1}{2*3}+\frac{1}{3*4}-...= 1 <br />

So i guess there must be a trick for all of them using the expansions

edit: I am in the same class as this dude ;P we're talkin about it if you're confused
 
yeah ^ what he said
 
How about multiplying log(1+x) by x and then integrating?
 
vela said:
How about multiplying log(1+x) by x and then integrating?

well I get;

\frac{x^{3}}{3} - \frac{x^{4}}{4}+\frac{x^{5}}{5}-...

Which I can't see being very helpful...anyway you get a weird integral on the LHS of

\frac{x^{2}log(1+x)}{2}+integralof(\frac{x^{2}}{2(1+x)})

Maybe I'm being slow...but can you give me a bit more of a hint? ;s
 
It would help if I integrated correctly. :wink:

Try integrating just log(1+x).
 
vela said:
It would help if I integrated correctly. :wink:

Try integrating just log(1+x).

Ah that's quite clever! ( i realized i integrated wrong before) Using it I was able to do the 2nd one, but not the first :S. (If you let x = -1 the LHS is messed up...)
 
Last edited:
Also the next part is show that;

\frac{1}{1!}+\frac{4}{2!}+\frac{9}{3!}+\frac{16}{4!}...=2e

following on i tried messing round with integrals but came to nothing :( any advice would be great
 
  • #10
You could try taking the limit as x approaches -1+.
 
  • #11
You might have noticed by integrating, you managed to get a factor of n on the bottom of each term. What do you get when you differentiate ex? Can you see how you might combine a sequence of differentiating and multiplying by powers of x to get the series you want?
 
  • #12
Yeah i worked it out about 20 minutes ago!

Thanks a lot for the help buddy, it's much obliged
 

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