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

Is it correct.

  1. Jun 12, 2013 #1
    Prove that if an element is in the union of two infinite sets then it is not necessarily in their intersection:

    Proof: ImageUploadedByPhysics Forums1371044407.002411.jpg

    Have I solved it correctly?
  2. jcsd
  3. Jun 12, 2013 #2
    Looks good.
  4. Jun 12, 2013 #3


    User Avatar
    Science Advisor

    Correct but awkward. The best way to prove something is NOT true is to give a counter-example. Let A= {(x, 0)}, the set of all points on the x-axis. Let B= {(0, y)}, the set of all points on the y-axis. Those are both infinite sets and their union is the set of all pairs of numbers in which at least one of the pair is 0. But their intersection is just {(0, 0)}.
  5. Jun 16, 2013 #4
    I don't see the OP's method being any more or less awkward than yours, really.

    That is exactly what the OP did; just using a different couple of sets. The main difference was that the OP's two sets didn't even intersect at all, which is fine.

    In fact, you could argue that that is a more obvious way to find a counter-example. Surely it is easier to find two infinite sets that do not intersect in the first place?

    However, I'm not saying your answer was awkward either. :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook