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Theorem dfbi3 1018
 Description: An alternate definition of the biconditional. Theorem *5.23 of [WhiteheadRussell] p. 124. (Contributed by NM, 27-Jun-2002.) (Proof shortened by Wolf Lammen, 3-Nov-2013.) (Proof shortened by NM, 29-Oct-2021.)
Assertion
Ref Expression
dfbi3 ((𝜑𝜓) ↔ ((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)))

Proof of Theorem dfbi3
StepHypRef Expression
1 con34b 305 . . 3 ((𝜓𝜑) ↔ (¬ 𝜑 → ¬ 𝜓))
21anbi2i 730 . 2 (((𝜑𝜓) ∧ (𝜓𝜑)) ↔ ((𝜑𝜓) ∧ (¬ 𝜑 → ¬ 𝜓)))
3 dfbi2 661 . 2 ((𝜑𝜓) ↔ ((𝜑𝜓) ∧ (𝜓𝜑)))
4 cases2 1016 . 2 (((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)) ↔ ((𝜑𝜓) ∧ (¬ 𝜑 → ¬ 𝜓)))
52, 3, 43bitr4i 292 1 ((𝜑𝜓) ↔ ((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)))
 Colors of variables: wff setvar class Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 196   ∨ wo 382   ∧ wa 383 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 197  df-or 384  df-an 385 This theorem is referenced by:  pm5.24  1020  4exmidOLD  1022  nanbi  1494  ifbi  4140  sqf11  24910  bj-dfbi4  32683  raaan2  41496  2reu4a  41510
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