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Math Help - problem finding an equality that holds between set quantities

  1. #1
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    Exclamation problem finding an equality that holds between set quantities

    I have just started studying sets and am not sure how to go about this problem.

    It says "find an equality that holds between the quantities #A, #B, #(AUB), #(A-intersection-B)"

    I am not sure if this is correct because the answer I have come up with seems so simple, but if A and B were the same sets (or A a subset of B, both of them having exactly the same elements), wouldn't #(AUB) (the number of elements in AUB) be the same as #A, #B and #(A intersection B)???

    Thanks a lot!
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  2. #2
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    \#(A\cup B)=\#A+\#B-\#(A\cap B)
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  3. #3
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    Great, thanks so much!
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