My Math Forum Cardinality of union of two sets proof

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August 16th, 2017, 03:59 AM   #11
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 Originally Posted by MATHEMATICIAN Seems like I did not know about the correct notation
The notation isn't what matters here... If you defined $o(A)$ to mean what $n(A \setminus B)$ means, it would have been fine. But you defined $o(A)$ to be identically zero.

 August 16th, 2017, 04:33 AM #12 Math Team     Joined: Jul 2013 From: काठमाडौं, नेपाल Posts: 878 Thanks: 60 Math Focus: सामान्य गणित I said so because there were only two sets in his example.
August 16th, 2017, 05:31 AM   #13
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Quote:
 Originally Posted by MATHEMATICIAN I said so because there were only two sets in his example.
Ah, then perhaps you should have said "o (A) = number of elements in A that are not in B" rather than "o (A) = number of elements that belong to only A (should not belong to other sets)". They are very different statements!

August 16th, 2017, 06:19 AM   #14
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 Originally Posted by cjem Ah, then perhaps you should have said "o (A) = number of elements in A that are not in B" rather than "o (A) = number of elements that belong to only A (should not belong to other sets)". They are very different statements!
Perhaps, I should have done so.

 Tags cardinality, proof, sets, union

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