Theorem necon4i 2577

Description: Contrapositive inference for inequality. (Contributed by NM, 17-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.)

Hypothesis

Ref	Expression
necon4i.1	⊢ (A ≠ B → C ≠ D)

Assertion

Ref	Expression
necon4i	⊢ (C = D → A = B)

Proof of Theorem necon4i

Step	Hyp	Ref	Expression
1		necon4i.1	. . 3 ⊢ (A ≠ B → C ≠ D)
2	1	necon2bi 2563	. 2 ⊢ (C = D → ¬ A ≠ B)
3		nne 2521	. 2 ⊢ (¬ A ≠ B ↔ A = B)
4	2, 3	sylib 188	1 ⊢ (C = D → A = B)

Syntax hints:  ¬ wn 3   → wi 4   = wceq 1642   ≠ wne 2517

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-ne 2519

This theorem is referenced by:  pw10b  4167  map0  6026