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Proposition: For all formulas A and B, the following holds:
Proof: by truth table.
~(A /\ B) iff ~A \/ ~B, ~(A \/ B) iff ~A /\ ~B.
Consequence: For all formulas A and B, we have:
A \/ B iff ~(~A /\ ~B).Proof: by inversion of negation.
Disjunction can thus be defined by conjunction and negation.