1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Series and convergence/divergence help

  1. Mar 28, 2007 #1
    Hey all, I am really struggling to understand this chapter about series. These are a few problems about convergence and divergence, and I'll probably have some questions about Taylor and maclaurin series when I do those problems too.

    1. The problem statement, all variables and given/known data
    1. Determine whether the sequence is convergent or divergent. If it is convergent, find its limit.

    a_n = 9^(n+1) / 10^n

    Determine whether the series is convergent or divergent:
    2. 1 / n(ln n)^2, series starts at n=2 and goes to infinity

    3. Find the sum of the series:
    [arctan (n+1) - arctan n], series starts at n=1 and goes to infninity.

    2. Relevant equations

    3. The attempt at a solution
    1. I broke the problem down to: 9(9/10)^n, and said that as n->infinity the sequence also goes to infinity, so it's divergent. Need my method checked on that one.

    2. Don't know about this one

    3. Examples I've seen of these kinds of problems end up being a geometric series, so I just use a/1-r to find out sum of the series. But I don't think this applies here, so what else should I do?

    Thanks for any help :smile:
  2. jcsd
  3. Mar 29, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Are you sure about this. For instance, the first two terms are 9*9/10=8.1, 9*81/100=7.29. The sequence seems to be decreasing. Indeed, if you have a number 'a' such that -1<a<1, then the result of raising 'a' to any power greater than one results in a number lesser than 'a'.

    I would be surprised if you were asked this question and nowhere in your textbook or class notes it was mentioned that the limit of a^n is 0 for any a such that -1<a<1.
  4. Mar 29, 2007 #3
    1.the first series is pretty easy, it converges and its limit is 0.
    evaluate it using dallamber's test, or criterion. DO you know what it states?
    2. for the second one i did not solve it all, but i think that considering that the series 1/ln*n diverges, than i think that considering this you can find if your serie converges or diverges!! So i think the comparison test will work on second one. As for the third i have no idea.
    Last edited: Mar 29, 2007
  5. Mar 29, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    For the third one, just write out the first few terms and scratch your head.
  6. Mar 29, 2007 #5


    User Avatar
    Science Advisor
    Homework Helper

    And the second one needs an integral test.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook