Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Limit Proof Confusion

  1. Sep 16, 2014 #1
    Hello Folks,

    I am solving a limit proof that has the following attached solution.

    Question: Assume all sn ≠ 0. and that the limit L = lim abs(sn+1/sn) exists. Show that if L<1. the lim sn=0

    I Understand the solution except for one part which is also attached..

    sn = sN*sN+1/sN*** sn/sn-1

    Can someone please explain this portion of the problem? I don't understand how sN+2 cancels with sn-1.. It definitely has something to do with n≥N but how they derived it I am not sure


    Attached Files:

    Last edited: Sep 16, 2014
  2. jcsd
  3. Sep 16, 2014 #2
    It only cancels when n = N + 3. When n = N + 4 there is another term in the ..... which cancels with the n-1 term, and so on.
  4. Sep 16, 2014 #3
    Thank you for the response, but I still don't quite understand..
  5. Sep 17, 2014 #4
    Try writing out the terms for N = 0, n = 3.
  6. Sep 17, 2014 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Another way to look at it for n > N is to think of n = N + k, then
    [tex]s_n = s_{N+k} = s_N \cdot \frac{s_{N+1}}{s_N} \cdot \frac{s_{N+2}}{s_{N+1}} \dots \frac{s_{N+k-1}}{s_{N+k-2}} \cdot \frac{s_{N+k}}{s_{N+k-1}}[/tex]
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook