1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A proof

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data
    prove that if a is less than zero , and if b is less than zero then ab is greater than zero.

    I have been having troubles with this problem.
    thanks.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 19, 2010 #2

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What have you tried so far? Do you know what axioms for positivity you can use?
     
  4. Sep 19, 2010 #3
    Im reading the subject in the book of Spivak and frankly I dont understand what he says.
    on page 12 he defined P to be a positive number thenhe said that for a number a only one of this three equalities is correct

    a=0, a is a is part of P, and - a is part of P. I dont understand the last one since he defined P as the set of all the positive numbers maybe there might be a mistake in my book though.

    is there any other way to prove this theorem?
     
  5. Sep 19, 2010 #4

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    For example, if a=-3, then -a is in P, not a.

    You're going to have to use his definition of positivity to do the problem. You can't prove that something has a certain property without using its defining features!

    As a starting point: We know that a<0 and b<0 here, so (-a)>0 and (-b)>0. Try to work from here
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A proof
  1. Proof of Convergence (Replies: 3)

  2. Trig Proof (Replies: 4)

  3. Geometrical Proof (Replies: 6)

  4. "Arithmetic?" proof (Replies: 3)

Loading...