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Summation of series using method of difference

  • Thread starter hhm28
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  • #1
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Homework Statement


Here's my question. My school recently taught me finding summation using method of difference and what my teacher taught was just involving 2 partial fractions.

But this question appeared in my exercise given by my teacher.

r th term: (2r-1)/r(r+1)(r+2). Find summation of n th terms begin with r=1.

Can someone show me how to solve this using method of difference?


2. The attempt at a solution
What I got is sum (- 1/2r + 3/r+1 - 5/2(r+2) ).

And I stucked. What I used to know is r th term is f(r) - f(r-1). and sum of rth term is f(n)- f(0).
I cant get the f(r) - f(r-1) either because -1/2r and -5/2(r+2) both having the same sign.
 

Answers and Replies

  • #2
tiny-tim
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welcome to pf!

hi hhm28! welcome to pf! :smile:
r th term: (2r-1)/r(r+1)(r+2). Find summation of n th terms begin with r=1.
might be easier to split it up first

for example 2/(r+1)(r+2) - 1/r(r+1)(r+2) :wink:
 
  • #3
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This did help me. But the problem is i stucked somewhere.

I got sigma -[(2/r+2) - (3/r+1) + (1/2r+2) + (1/2r)] and I stucked.

I should've make it into f(r)-f(r-1). =( I'm stupid Argh....

Can you show me how? Perhaps upload a photo. LOL
 
  • #4
tiny-tim
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hint: how would you do ∑ 1/r(r+1) ?

use the same method for ∑ 1/r(r+1)(r+2) :smile:
 
  • #5
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LOL. Thats the problem. I only know how to solve f(r) - f(r-1).

Now there is 3 partial fractions. But with 2/(r+1)(r+2) - 1/r(r+1)(r+2), i managed to get ∑ -[(2/r+2) - (3/r+1) + (1/2r+2) + (1/2r)].
 
  • #6
tiny-tim
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ok, so how would you do ∑ 1/r(r+1) ? :smile:
 
  • #7
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1/r(r+1) = (1/r) - (1/r+1)

∑ 1/r(r+1) = ∑ (1/r) - (1/r+1)
= -∑ [(1/r+1) -(1/r)]

Let f(r)= 1/ r+1 , f(r-1)= 1/r
∑ 1/r(r+1) = -∑ f(r) - f(r-1)
= - [f(n) - f(0)]
= - [(1/n+1) - 1]
= n/n+1
 
  • #8
tiny-tim
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1/r(r+1) = (1/r) - (1/r+1)
ok, now do the same for ∑ 1/r(r+1)(r+2) :smile:
 
  • #9
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∑1/r(r+1)(r+2) = ∑1/2r - 1/(r+1) + 1/2(r+2)

How do i continue? the 1/(r+1) is disrupting. =(
 
  • #10
tiny-tim
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∑1/r(r+1)(r+2) = ∑1/2r - 1/(r+1) + 1/2(r+2)
no, try double fractions like 1/r(r+1) :wink:
 
  • #11
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means that A/r(r+1) + B/(r+2) to get partial?
 
  • #12
tiny-tim
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A/r(r+1) + B/(r+1)(r+2) :smile:
 
  • #13
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Gosh. I never learned that =(
 
  • #14
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Thank you so much anywhere. =)
 
  • #15
vela
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Homework Statement


Here's my question. My school recently taught me finding summation using method of difference and what my teacher taught was just involving 2 partial fractions.

But this question appeared in my exercise given by my teacher.

r th term: (2r-1)/r(r+1)(r+2). Find summation of n th terms begin with r=1.

Can someone show me how to solve this using method of difference?


2. The attempt at a solution
What I got is sum (- 1/2r + 3/r+1 - 5/2(r+2) ).

And I stucked. What I used to know is r th term is f(r) - f(r-1). and sum of rth term is f(n)- f(0).
I cant get the f(r) - f(r-1) either because -1/2r and -5/2(r+2) both having the same sign.
Good start. Try writing out the first few terms:

r=1: ##-\frac{1}{2}\left(\frac{1}{1}\right) + 3\left(\frac{1}{2}\right) - \frac{5}{2}\left(\frac{1}{3}\right)##

r=2: ##-\frac{1}{2}\left(\frac{1}{2}\right) + 3\left(\frac{1}{3}\right) - \frac{5}{2}\left(\frac{1}{4}\right)##

r=3: ##-\frac{1}{2}\left(\frac{1}{3}\right) + 3\left(\frac{1}{4}\right) - \frac{5}{2}\left(\frac{1}{5}\right)##

r=4: ##-\frac{1}{2}\left(\frac{1}{4}\right) + 3\left(\frac{1}{5}\right) - \frac{5}{2}\left(\frac{1}{6}\right)##

r=5: ##-\frac{1}{2}\left(\frac{1}{5}\right) + 3\left(\frac{1}{6}\right) - \frac{5}{2}\left(\frac{1}{7}\right)##

Now look at the pieces with (1/3) (or (1/4) or (1/5)). Can you see a pattern to how the various parts cancel?
 
Last edited:

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