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Theorem pm5.21 834
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  |-  ( ( -.  ph  /\  -.  ps )  ->  ( ph  <->  ps )
)

Proof of Theorem pm5.21
StepHypRef Expression
1 pm5.21im 340 . 2  |-  ( -. 
ph  ->  ( -.  ps  ->  ( ph  <->  ps )
) )
21imp 420 1  |-  ( ( -.  ph  /\  -.  ps )  ->  ( ph  <->  ps )
)
Colors of variables: wff set class
Syntax hints:   -. wn 5    -> wi 6    <-> wb 178    /\ wa 360
This theorem is referenced by:  oibabs  856  reusv7OLD  4437  onsuct0  24054
This theorem was proved from axioms:  ax-1 7  ax-2 8  ax-3 9  ax-mp 10
This theorem depends on definitions:  df-bi 179  df-an 362
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