Theorem unass 3316

Description: Associative law for union of classes. Exercise 8 of [TakeutiZaring] p. 17. (Contributed by NM, 3-May-1994.) (Proof shortened by Andrew Salmon, 26-Jun-2011.)

Assertion

Ref	Expression
unass	|- ( ( A u. B ) u. C ) = ( A u. ( B u. C ) )

Proof of Theorem unass

Dummy variable x is distinct from all other variables.

Step	Hyp	Ref	Expression
1		elun 3300	. . 3 |- ( x e. ( A u. ( B u. C ) ) <-> ( x e. A \/ x e. ( B u. C ) ) )
2		elun 3300	. . . 4 |- ( x e. ( B u. C ) <-> ( x e. B \/ x e. C ) )
3	2	orbi2i 763	. . 3 |- ( ( x e. A \/ x e. ( B u. C ) ) <-> ( x e. A \/ ( x e. B \/ x e. C ) ) )
4		elun 3300	. . . . 5 |- ( x e. ( A u. B ) <-> ( x e. A \/ x e. B ) )
5	4	orbi1i 764	. . . 4 |- ( ( x e. ( A u. B ) \/ x e. C ) <-> ( ( x e. A \/ x e. B ) \/ x e. C ) )
6		orass 768	. . . 4 |- ( ( ( x e. A \/ x e. B ) \/ x e. C ) <-> ( x e. A \/ ( x e. B \/ x e. C ) ) )
7	5, 6	bitr2i 185	. . 3 |- ( ( x e. A \/ ( x e. B \/ x e. C ) ) <-> ( x e. ( A u. B ) \/ x e. C ) )
8	1, 3, 7	3bitrri 207	. 2 |- ( ( x e. ( A u. B ) \/ x e. C ) <-> x e. ( A u. ( B u. C ) ) )
9	8	uneqri 3301	1 |- ( ( A u. B ) u. C ) = ( A u. ( B u. C ) )

Syntax hints:   \/ wo 709   = wceq 1364   e. wcel 2164   u. cun 3151

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-io 710  ax-5 1458  ax-7 1459  ax-gen 1460  ax-ie1 1504  ax-ie2 1505  ax-8 1515  ax-10 1516  ax-11 1517  ax-i12 1518  ax-bndl 1520  ax-4 1521  ax-17 1537  ax-i9 1541  ax-ial 1545  ax-i5r 1546  ax-ext 2175

This theorem depends on definitions:  df-bi 117  df-tru 1367  df-nf 1472  df-sb 1774  df-clab 2180  df-cleq 2186  df-clel 2189  df-nfc 2325  df-v 2762  df-un 3157

This theorem is referenced by:  un12  3317  un23  3318  un4  3319  qdass  3715  qdassr  3716  rdgisucinc  6438  oasuc  6517  unfidisj  6978  undifdc  6980  djuassen  7277  fzosplitprm1  10301  hashunlem  10875  plyun0  14882