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Proof of De Morgan's law

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Homework Statement


I am trying to prove (~P & ~Q) => ~(P V Q) using only MP, negation elimination, &I conjunction, &E simplification, addition, constructive dilemma, biconditional introduction and elimination, conditional proof, and reductio ad absurdum.


Homework Equations





The Attempt at a Solution


It seems to me that the only way to do the prove is reductio ad absurdum. But,
I am stuck after hypothesizing (P V Q). I looked at embedding other hypothesis to get a useful negation but could only come up with ~(P & Q). I don't think it will be useful.

Any hints on the hypothesis I need to use?
 

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