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Theorem difex2 7011
 Description: If the subtrahend of a class difference exists, then the minuend exists iff the difference exists. (Contributed by NM, 12-Nov-2003.) (Proof shortened by Andrew Salmon, 12-Aug-2011.)
Assertion
Ref Expression
difex2 (𝐵𝐶 → (𝐴 ∈ V ↔ (𝐴𝐵) ∈ V))

Proof of Theorem difex2
StepHypRef Expression
1 difexg 4841 . 2 (𝐴 ∈ V → (𝐴𝐵) ∈ V)
2 ssun2 3810 . . . . 5 𝐴 ⊆ (𝐵𝐴)
3 uncom 3790 . . . . . 6 ((𝐴𝐵) ∪ 𝐵) = (𝐵 ∪ (𝐴𝐵))
4 undif2 4077 . . . . . 6 (𝐵 ∪ (𝐴𝐵)) = (𝐵𝐴)
53, 4eqtr2i 2674 . . . . 5 (𝐵𝐴) = ((𝐴𝐵) ∪ 𝐵)
62, 5sseqtri 3670 . . . 4 𝐴 ⊆ ((𝐴𝐵) ∪ 𝐵)
7 unexg 7001 . . . 4 (((𝐴𝐵) ∈ V ∧ 𝐵𝐶) → ((𝐴𝐵) ∪ 𝐵) ∈ V)
8 ssexg 4837 . . . 4 ((𝐴 ⊆ ((𝐴𝐵) ∪ 𝐵) ∧ ((𝐴𝐵) ∪ 𝐵) ∈ V) → 𝐴 ∈ V)
96, 7, 8sylancr 696 . . 3 (((𝐴𝐵) ∈ V ∧ 𝐵𝐶) → 𝐴 ∈ V)
109expcom 450 . 2 (𝐵𝐶 → ((𝐴𝐵) ∈ V → 𝐴 ∈ V))
111, 10impbid2 216 1 (𝐵𝐶 → (𝐴 ∈ V ↔ (𝐴𝐵) ∈ V))
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 196   ∧ wa 383   ∈ wcel 2030  Vcvv 3231   ∖ cdif 3604   ∪ cun 3605   ⊆ wss 3607 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1762  ax-4 1777  ax-5 1879  ax-6 1945  ax-7 1981  ax-8 2032  ax-9 2039  ax-10 2059  ax-11 2074  ax-12 2087  ax-13 2282  ax-ext 2631  ax-sep 4814  ax-nul 4822  ax-pr 4936  ax-un 6991 This theorem depends on definitions:  df-bi 197  df-or 384  df-an 385  df-tru 1526  df-ex 1745  df-nf 1750  df-sb 1938  df-clab 2638  df-cleq 2644  df-clel 2647  df-nfc 2782  df-ral 2946  df-rex 2947  df-rab 2950  df-v 3233  df-dif 3610  df-un 3612  df-in 3614  df-ss 3621  df-nul 3949  df-sn 4211  df-pr 4213  df-uni 4469 This theorem is referenced by:  elpwun  7019
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