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Homework Help: Proving If GCD(a,b) = c Then c^2/ab

  1. Mar 18, 2012 #1
    So here is the problem.

    Prove that If the gcd(a,b) = c then c^2 divides ab

    I know it looks very simple and it seems to be true, But I get the feeling i'm doing something wrong here in my proof. Would appreciate it if someone can explain if I'm on the right track or not.


    Proof by Contradiction:

    Assume that gcd(a,b) = c and c^2 does not divide ab
    Let a = 6 and b = 9. So,

    gcd(6,9) = 3
    ab = 54
    c^2 = 9

    But 54/9 = 6, so 9 divides 54 and therefore c^2 divides ab. This contradicts the assumption, so the claim "If gcd(a,b) = c then c^2/ab" is infact true.

  2. jcsd
  3. Mar 18, 2012 #2
    Unfortunately, you are right to be uneasy about your proof! What you have shown is that for a = 6 and b = 9 then the statement is true. However, there remain an infinite number of unconsidered cases. What if a = 3 and b = 8? a = 444 and b = 86274?

    Instead of considering specific numbers, then, we just leave a and b as arbitrary numbers. That way if we prove the statement for arbitrary numbers then we have done all cases.

    So we have gcd(a,b) = c. Then c divides a and c divides b. So cx = a and cy = b. Can you see how the rest of it goes?

    Hope that helps!
    Last edited: Mar 18, 2012
  4. Mar 18, 2012 #3
    This is very new to me. I get what you mean about how it doesn't prove it for all integers, and I do understand how a = cx and b = cy. But I'm struggling to see where to go next. could I say that a*b = cx*cy = c^2xy? And then c^2xy must be divisible by c^2?
  5. Mar 18, 2012 #4
    That is how I would do it. Unless you cannot assume multiplicative commutativity, i.e. a * b = b * a (if your professor has not mentioned anything like that, chances are you can).
  6. Mar 18, 2012 #5
    No I don't think anything like that was mentioned. That's got to be it! Thanks so much for your help I appreciate it :)
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