Theorem undifabs 3601

Description: Absorption of difference by union. (Contributed by NM, 18-Aug-2013.)

Assertion

Ref	Expression
undifabs	|- ( A u. ( A \ B ) ) = A

Proof of Theorem undifabs

Step	Hyp	Ref	Expression
1		ssid 3268	. . 3 |- A C_ A
2		difss 3355	. . 3 |- ( A \ B ) C_ A
3	1, 2	unssi 3404	. 2 |- ( A u. ( A \ B ) ) C_ A
4		ssun1 3392	. 2 |- A C_ ( A u. ( A \ B ) )
5	3, 4	eqssi 3264	1 |- ( A u. ( A \ B ) ) = A

Syntax hints:   = wceq 1402   \ cdif 3217   u. cun 3218

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-in1 623  ax-in2 624  ax-io 721  ax-5 1500  ax-7 1501  ax-gen 1502  ax-ie1 1546  ax-ie2 1547  ax-8 1557  ax-10 1558  ax-11 1559  ax-i12 1560  ax-bndl 1562  ax-4 1563  ax-17 1579  ax-i9 1583  ax-ial 1587  ax-i5r 1588  ax-ext 2220

This theorem depends on definitions:  df-bi 117  df-tru 1405  df-nf 1514  df-sb 1816  df-clab 2225  df-cleq 2231  df-clel 2234  df-nfc 2381  df-v 2823  df-dif 3222  df-un 3224  df-in 3226  df-ss 3233

This theorem is referenced by:  exmid1stab  4340