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: Limit theorems and determining convergence

  1. Aug 25, 2011 #1
    1. The problem statement, all variables and given/known data


    3. The attempt at a solution

    I'm having some trouble getting my head around these 3 problems. Any ideas on how to approach them are welcome.
  2. jcsd
  3. Aug 25, 2011 #2
    Can you show us what you have tried?

    You should be able to simplify the expressions in a) and c).

    For b), take a look at the numerator and see what the values for the first few n are. Notice that n! is always an integer, so what can you determine about the possible values of sin(n! * pi /3) ? How does the (-1)^n affect those possible values? Can you determine a maximum possible value and a minimum possible value for the numerator? If so, what will happen to the value of the whole expression as n gets larger in the denominator?
  4. Aug 25, 2011 #3
    The first few limits are straightforward. For part a), algebraic manipulation will get you to the correct answer (it does exist; I'll let you figure out what it is :). Try to think of how you can use the various powers of n. For part b), clearly, as n! gets larger, it contains a factor of 3 which cancels with the 3 in the denominator of the sine function. Thus, the sine function goes to zero and the sandwich theorem can be applied with ease.

    As an edit, think about how you can simplify the third expression to get a limit that is far more intuitive to work with. After simplification, the limit will follow naturally.
    Last edited: Aug 26, 2011
  5. Aug 27, 2011 #4
    Thanks for the replies!

    I'm having some trouble manipulating the denominator of a).

    For b), I can see that whenever n>3 the sin term is 0. Since the (-1)^n-1 oscillates, does the expression diverge by unbounded oscillation?

    For c), I can simplify it up to x^(3 - logx). After that I can't seem to go any further.
  6. Aug 27, 2011 #5

    Char. Limit

    User Avatar
    Gold Member

    No, as the oscillation isn't unbounded. It clearly tends to a specific number. Try neglecting the sine term (as you can for n>3), and just look at (-1)^(n-1)/(n+1). As n increases, what does this get closer to?
  7. Aug 27, 2011 #6
    Can it be... 0?!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook