s E T" is a tautology. This can be proved (or disproved) using a truth table. 2) RU(SnT) = (RUS)n(RUT). 3 Proofs in Set Theory 45 In writing the table, it is implicitly assumed that a general element x is being considered; the notation T (or F) under the name of a set means that the statement that x is a member of that set is true (or false). 2) is a tautology provided the last two columns are equal, which is seen to be true.
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