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Homework Help: Cancellation property of addition of natural numbers

  1. Mar 23, 2009 #1
    I have to prove that for all k,m,n [tex]\in[/tex] [tex]N[/tex] that if m+k = n+k, then m=n.

    The problem mentions that I must prove this by induction.

    I did the base case k = 0: If m+0 = n+0, by identity m=n.

    Then I attempt to show that m+1 = n+1 implies m=n, but I am stuck, I don't see how induction can be used to prove this and then next predicate that m+k = n+k implies m=n.

    Any help would be greatly appreciated, and thanks in advance.
  2. jcsd
  3. Mar 23, 2009 #2


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    Staff: Mentor

    "show that m+1 = n+1 implies m=n"

    Can you subtract 1 from both sides? Not sure if that's allowed in this step of the proof. It's less general than subtracting k from both sides of the original question, but not by much...
  4. Mar 24, 2009 #3


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    Staff Emeritus
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    The problem didn't say you had to induct on k, though.

    As an aside... doesn't
    m+k = n+k implies m = n​
    follow straightforwardly from
    m+1 = n+1 implies m = n​
  5. Mar 24, 2009 #4


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    What is your definition of addition for natural numbers?
  6. Mar 24, 2009 #5
    I just don't get this. This is exactly the problem I have with some of the "proofs" that I am given to construct in textbooks, they are obvious before the fact, and that is a horrible way to teach a concept.

    m + k = n + k | -k
    m = n

    Of course that is true! there is no natural number k so that 3 + k = 4 + k!

    I have a mental block or something which prohibits me from learning a method by examples where I don't actually need to use the method to get the desired result!


  7. Mar 24, 2009 #6
    If you're allowed to, use the fact there is an ordering on the natural numbers.
  8. Mar 24, 2009 #7


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    Homework Helper

    i know what you mean kenewbie, i'm struggling with the same issue on analysis proofs... but there in lies the twist, if you can't prove something that appears obvious, how can you prove something less obvious, that probably relies on your obvious friend?

    i think its about breaking it right down to what actually is assumed eg. your axioms... and what follows as a logical consequence from those axioms alone. Otherwise seemingly simple assumptions could get swept up in your reasoning without a concrete base for them

    probably wrong place to hijack somones question to discuss it, but interested on anyone else's thoughts on this
  9. Mar 24, 2009 #8
    Well, your first example uses k = 0. Your next step you said that your k was 1 greater than your previous value of k, k = k + 1. If you prove that this next step is also true and that this process of getting to the next natural number is valid, then you just proved it, I think. I haven't done much of this kind of problem.
  10. Mar 25, 2009 #9


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    I asked before, what definition of addition are you using? I don't see how you can prove anything about addition of natural numbers without using the basic definition.
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