Theorem pm2.73 818

Description: Theorem *2.73 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.)

Assertion

Ref	Expression
pm2.73	⊢ ((φ → ψ) → (((φ ∨ ψ) ∨ χ) → (ψ ∨ χ)))

Proof of Theorem pm2.73

Step	Hyp	Ref	Expression
1		pm2.621 397	. 2 ⊢ ((φ → ψ) → ((φ ∨ ψ) → ψ))
2	1	orim1d 812	1 ⊢ ((φ → ψ) → (((φ ∨ ψ) ∨ χ) → (ψ ∨ χ)))

Syntax hints:   → wi 4   ∨ wo 357

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-or 359  df-an 360

This theorem is referenced by: (None)