Prove that A (A B)’ = ϕ

LHS = A (A B)’


Using De-Morgan’s law (A B)’ = (A’ B’)


LHS = A (A’ B’)


LHS = (A A’) (A B’)


We know that A A’ = ϕ


LHS = ϕ (A B’)


We know that intersection of null set with any set is null set only


Let (A B’) be any set X hence


LHS = ϕ X


LHS = ϕ


LHS = RHS


Hence proved


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