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Epsilon Delta proof

  1. Feb 18, 2008 #1
    I’m going to say from the beginning that I need to hand this problem in. I'm not looking for the answer, I think I already have it, just want a critical eye.

    I need someone to look over this problem and tell me if it's good. Not just if it's right but if it's perfect. I always get the problem right then get minus points because I didn't explain it enough.

    So here it is

    Give an ε-δ proof of
    lim x->a of ((3x²-3a²)/(x-a)) = 6a

    Proof: Note |f(x) – a| = |f(x) – 6a| = | (3x²-3a²-6ax-6a²)/(x-a) | = | (3(x²-2ax+a²)/(x-a)) | = |3x-3a|

    |3x-3a|<ε (get rid of 3 to make it smaller) |x-a|<ε when |x-a|<δ let δ=ε


    Given ε>δ let δ=ε

    0<|x-a|<δ then
    |f(x) – a| = |3x-3a|<δ=ε

    Therefore
    lim x->a of ((3x²-3a²)/(x-a)) = 6a
     
  2. jcsd
  3. Feb 18, 2008 #2
    What you really need to say is let d = e/3
    Then you get |x-a|<d ==> |3x-3a|< 3*d =e, which is what you needed.
    Up to there it was fine though
     
  4. Feb 18, 2008 #3
    ooo Thanks. That was a pretty large mistake.
     
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