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Homework Help: Prove sequence diverges to infinity

  1. Sep 29, 2012 #1
    1. The problem statement, all variables and given/known data

    I have to prove that the sequence a(n)=(n^3-n +1)/(2n+4) diverges to infinity.

    2. Relevant equations

    3. The attempt at a solution

    Observe that n^3-n +1 > (1/2)n^3 and 2n+4≤4n in n≥2

    I am now stuck on how to proceed. I am confused on opposite inequalities for the numerator and denom. Can you direct as to how I'm to proceed?

  2. jcsd
  3. Sep 29, 2012 #2


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    Homework Helper

    To "see" it, just observe that for large n, the numerator approaches n3, while the denominator approaches 2n.

    To prove it, just do the long division to get a quadratic quotient (and a remainder, which vanishes at the limit). Or use synthetic division. From this point on, the limit should be obvious (although you can complete the square for the quadratic to make it even more rigorous).
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