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Homework Help: Is my proof correct? (show that if a < 0, then -a > 0)

  1. Jun 3, 2012 #1

    Show that if a < 0, hen -a > 0

    2. Relevant equations


    3. The attempt at a solution:

    Assume the result is false.

    Thus, by trichotomy:


    -a = 0

    This is not possible. A real number can be greater than zero, less than zero or equal to zero.
    Since it has been given that -a > 0, it follows that -a ≠ 0


    -a < 0

    If -a < 0, then a > 0

    a + (-a) < 0 + a

    [itex]\Rightarrow[/itex] a > 0, which is not true!

    Since, it is given that a < 0


    This is my first foray into learning calculus in a more rigorous fashion. I've never done any proofs before. This seems okay to me but it is possible that there is something wrong somewhere. So, is this indeed correct or did I miss something?

    Thank you
  2. jcsd
  3. Jun 3, 2012 #2
    Hi Mépris! :smile:

    You aren't given that fact. :uhh: That's what you are trying to prove..

    This part sounds correct!
  4. Jun 3, 2012 #3
    Hello Inifinitum!


    This is actually a two-part question but I didn't think posting the first part would be necessary. Here is the full question:
    "Show that if a > 0, then -a < 0; and if a < 0, then -a < 0"

    The solution to the first part was provided and it was said that the second part of the solution could be inferred from the first. So, I tried to prove it like the first part was proven and I didn't look closely enough. (complacency, I guess?)


    If anyone's wondering, the question is from Ian Craw's notes on "Advanced Calculus and Analysis", which is a first-year course in the math degree at the University of Aberdeen. I decided to start off my study of calculus with that because I found Courant (there is a free online copy on the AIT's website) a little too hard for my level and Craw's notes/book are written with people who've studied the A-Levels and Scottish Advanced Highers in mind. Since, I did A-Level Mathematics, these notes are a nice transition. :-) :-)



    The above is my second attempt, which looks correct. Nevertheless, I'm not certain about my wording. Is the way I write precise enough? Further, am I correct in saying that bit about a "non-zero real number..."? I ask because it was not explicitly stated in the definition given in the notes. After the definition, it is stated that "a real number is either positive, negative or zero". Can I just use that when talking about "trichotomy"?

    Thank you.
  5. Jun 3, 2012 #4
    Nooo! How can a non-zero real number be equal to zero!!?

    When you got, a=0, you could immediately say that since its already given,[itex] a < 0[/itex], and by identity [itex]0=0[/itex], hence[itex]-a \neq 0[/itex], causing a contradiction.
  6. Jun 3, 2012 #5
    Okay, that was really stupid of me - tried to rush through it and just added that there. I can actually feel my cheeks flushing, which I don't think I've ever felt before. :$

    Great. Thanks a lot!