Let A, B and C be sets. Prove that
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Feb23-12, 09:55 PM
Another perspective: you can maybe rigorize it by saying:
a in B or a in C , but a not in B , so we must have a in C:
Basically, as you rightly concluded, a must be in one
of B or C, but ,by assumption/construction, a is not in B,
so a must be in C.