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Nightmares with formal proofs in set theory

  1. Mar 26, 2006 #1
    I am having a nightmare trying to prove things in set theory.

    One of my homework problems is to prove that:

    Dom(R U S) = Dom(R) U Dom(S)

    but i have no idea how to really do this. my teacher never went over this stuff!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! IT'S SO AGGRAVATING!

    can anyone reference a good site or book on how to prove things in set theory, such as the domain, inverse of function only if the function is one-to-one, etc???

  2. jcsd
  3. Mar 26, 2006 #2


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    The basic way to prove A is a subset of B is: Let x be a member of A. Then use what ever the definition of A is to show that x also satisfies the definition of B: x is a member of B.

    Here, unfortunately, you haven't told us what R and S are and you haven't told us what "Dom" means. If R and S are functions I might guess that "Dom" means domain.
  4. Mar 26, 2006 #3
    R and S are relations, Dom means domain
  5. Mar 26, 2006 #4


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    Dearly Missed

    Suppose that (x,y) satisfies the relation RUS (i.e, (x,y) is in Dom(RUS)
    Then, (x,y) either satisfies the relation R (i.e, (x,y) is in Dom(R)), or (x,y) satisfies the relation S (i.e, (x,y) is in Dom(S))
    Thus, Dom(RUS) is contained within Dom(R)UDom(S).

    I'll leave to you to show that Dom(R)UDom(S) is contained within Dom(RUS).
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