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Theorem dcn 842
Description: The negation of a decidable proposition is decidable. The converse need not hold, but does hold for negated propositions, see dcnn 848. (Contributed by Jim Kingdon, 25-Mar-2018.)
Assertion
Ref Expression
dcn  |-  (DECID  ph  -> DECID  -.  ph )

Proof of Theorem dcn
StepHypRef Expression
1 notnot 629 . . . 4  |-  ( ph  ->  -.  -.  ph )
21orim2i 761 . . 3  |-  ( ( -.  ph  \/  ph )  ->  ( -.  ph  \/  -.  -.  ph ) )
32orcoms 730 . 2  |-  ( (
ph  \/  -.  ph )  ->  ( -.  ph  \/  -.  -.  ph ) )
4 df-dc 835 . 2  |-  (DECID  ph  <->  ( ph  \/  -.  ph ) )
5 df-dc 835 . 2  |-  (DECID  -.  ph  <->  ( -.  ph  \/  -.  -.  ph ) )
63, 4, 53imtr4i 201 1  |-  (DECID  ph  -> DECID  -.  ph )
Colors of variables: wff set class
Syntax hints:   -. wn 3    -> wi 4    \/ wo 708  DECID wdc 834
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-in1 614  ax-in2 615  ax-io 709
This theorem depends on definitions:  df-bi 117  df-dc 835
This theorem is referenced by:  stdcndc  845  stdcndcOLD  846  dcnn  848  pm5.18dc  883  pm4.67dc  887  pm2.54dc  891  imordc  897  pm4.54dc  902  annimdc  937  pm4.55dc  938  orandc  939  pm3.12dc  958  pm3.13dc  959  dn1dc  960  xor3dc  1387  dfbi3dc  1397  dcned  2353  gcdaddm  11984  prmdc  12129  pcmptdvds  12342
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