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Math Help - Simple Induction Problem

  1. #1
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    Simple Induction Problem

    tried this in so many ways... not getting anywhere, dont know why - fresh ideas helpful.

    "Use induction to prove the statement. Identify P(n) explicitly for the statement."
    "For all n e N, 4 | ((5^n)-1)"

    Use spoiler alerts please, I dont want a full answer, just a nudge in the right direction.
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  2. #2
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    Re: Simple Induction Problem

    Check that the statement is true for some small n (e.g. n = 1, n = 2). Show that if the statement is true for n = k, then it is true for n = k+1.
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  3. #3
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    Re: Simple Induction Problem

    Sorry, I should have specified what I've already attempted. I've made my base case and determined a value for P(1), assumed it to be true using induction hypothesis for P(k), and attempted to show it to be true for P(k+1). However, I never seem able to make P(k+1) true.

    hypothesis: P(k) : 4 | ((5^k)-1)

    conclusion: P(k+1) : 4 | ((5^(k+1))-1)

    after this, i blank. all my methods seem to fail. still need a nudge in the right direction :S
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  4. #4
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    Re: Simple Induction Problem

    frustrating
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  5. #5
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    Re: Simple Induction Problem

    We know that 4|P(k), and we want to show that 4|P(k+1).

    Hint: find P(k+1) - P(k). Must 4 divide this expression?
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  6. #6
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    Re: Simple Induction Problem

    ahh, touche.

    thanks a bunch!
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  7. #7
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    Re: Simple Induction Problem

    Quote Originally Posted by richard1234 View Post
    We know that 4|P(k), and we want to show that 4|P(k+1).
    No, no, no and NO! P(k) is a property of k, so for each positive integer k, P(k) is either true or false. You can't say that 4 divides true or false! By the way, the problem asks to identify P(n), and the first thing the OP should have done when showing what he/she had already attempted was to write P(n) explicitly.
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