Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple Proof

  1. Jun 22, 2008 #1
    Theorem: Suppose that x is real, then x^2 > or = 0.
    Incorrect Proof: Suppose not. Then x is real and x^2<0. Consider x=3. 9<0, a contradiction. Therefore x^2 >=0.
  2. jcsd
  3. Jun 23, 2008 #2
    I'm sorry, but what exactly is the brain teaser at hand here?
  4. Jun 23, 2008 #3
    Why the proof is incorrect, even though seemingly it looks like it is.
  5. Jun 23, 2008 #4
    I don't see how could this fool anyone... Given the assumption P -> -Q , --Q(3) does not give rise to P -> Q. One needs to show that the assumption P -> -Q gives rise to --Q (a statement for all x being considered, not just 3) and therefore conclude P -> Q by reductio ad absurdum.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook