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-N definition of limit

  1. Nov 6, 2013 #1
    ϵ-N definition of limit

    Use the ϵ-N definition of limit to prove that lim[(2n+1)/(5n-2)] = 2/5 as n goes to infinity.

    The way I do it is Let ∊ > 0 be given. Notice N ∈ natural number (N) which satisfies {fill this box later}< N. It follows that if n>=N, then n > {fill this box later}, so for such n, |(2n+1)/(5n-2)-2/5| = |9/(25n-10)| = 9/5|1/(5n-2)|

    I am supposed to get to a something that is less than ∊

    How to make this to less than ∊?
     
  2. jcsd
  3. Nov 6, 2013 #2

    Dick

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    You are over halfway there. If you want 9/5|1/(5n-2)|<ε, you want 1/|5n-2|<5ε/9. 5n-2 is positive so you can drop the absolute value. Now turn it into an inequality for n.
     
  4. Nov 6, 2013 #3
    Limit and Convergence

    Suppose a_n > 0 and b_n > 0 for all n in natural number (N). Also, lim a_n/b_n = 0 as n goes to infinity. Then the sum of a_n converges if and only if the sum of b_n converges ...both from 1 to infinity.

    My approach is that lim a_n/b_n = 0 means that there exists N in natural number (N) for which |a_n/b_n - 0| < 0 for all n >= N. Then 0 < a_n < 0. The sum of a_n from 1 to infinity is 0. So The sum of a_n from 1 to infinity is convergent.

    Is this proof that easy or I miss something?
     
  5. Nov 6, 2013 #4

    haruspex

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    The 'only if' part is clearly false. Pick a_n = 2^-n, b_n = 1. Should it have said lim a_n/b_n = c > 0?
     
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