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Homework Help: Proof by induction

  1. Jun 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Using mathematical induction, prove the inequality
    (1+a)^n >= 1+na for all a>-1 and all n

    2. Relevant equations

    3. The attempt at a solution
    Base case
    1+a >= 1+a. the inequality holds.

    I am struggling with the inductive step.
    by working backward, i multiply both sides by (1+a) and this gives me

    (1+a)^(k+1) >= (1+ka)(1+a)

    all i can think of at this point is that 1+a >= 0, but i dont think this helps me.

    working the other way, starting with n=k+1, i find

    (1+a)(1+a)^k>= (1+ka)+a

    would is be enough to state that (1+a)> a and thus the inequality holds?

    thanks in advance
  2. jcsd
  3. Jun 22, 2009 #2
    (1+ka)+a=1+(k+1)a,thus the inequality holds for n=k+1
  4. Jun 22, 2009 #3


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    Homework Helper

    Hey Easty, i think you were close with your first step, not to sure about your 2nd,

    how about this, so as usual start by assuming the case for n is true and trying to show n+1 is also true based on this:

    (1+a)^(n+1) = (1+a)*(1+a)^n >= (1+a)*(1+na) = (1 + a + na + na^2) = 1 + (n+1)a + na^2

    So now you have
    (1+a)^(n+1) >= 1 + (n+1)a + na^2

    any ideas from here?
  5. Jun 22, 2009 #4
    i had gotten to that stage, but thought it led no where.
    The only term that isnt part of the original equality is na^2, since a is >-1, a^2 >= 0, so couldnt the extra term add some arbitary large postive number such that the inequality wasnt true?
  6. Jun 22, 2009 #5


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    so if na^2 is always >=0 can you think of another inequality to write that will help get you close to the required form?
  7. Jun 23, 2009 #6
    I think i've got it
    na^2 >= 0

    add 1+(n+1)a to both sides

    1+(n+1)a+na^2 >= 1+(n+1)a


    (1+a)^(n+1) >= 1 + (n+1)a + na^2 >= 1 + (n+1)a

    (1+a)^(n+1) >=1 + (n+1)a as required

    i'm pretty sure this is correct. Thanks for all your help lanedance
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