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Theorem pm5.21 831
Description: Two propositions are equivalent if they are both false. Theorem *5.21 of [WhiteheadRussell] p. 124. (Contributed by NM, 21-May-1994.)
Assertion
Ref Expression
pm5.21 ((¬ φ ¬ ψ) → (φψ))

Proof of Theorem pm5.21
StepHypRef Expression
1 pm5.21im 338 . 2 φ → (¬ ψ → (φψ)))
21imp 418 1 ((¬ φ ¬ ψ) → (φψ))
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4  wb 176   wa 358
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8
This theorem depends on definitions:  df-bi 177  df-an 360
This theorem is referenced by:  oibabs  851
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