Another proof of De Morgan’s law

1.5

S?T =

{x | x ? S?T}

= {x | ?(x?(S?T))}

= {x | ?(x?S ? x?T)}

Use De Morgan’s law for logic

= {x | x ? S ? x ? T}

= {x | x ? S ? x ? T}

={x | x ? S?T}

End of proof

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