Theorem ax9 2159

Description: Proof of ax-9 2155 from ax9v1 2157 and ax9v2 2158, proving sufficiency of the conjunction of the latter two weakened versions of ax9v 2156, which is itself a weakened version of ax-9 2155. (Contributed by BJ, 7-Dec-2020.) (Proof shortened by Wolf Lammen, 11-Apr-2021.)

Assertion

Ref	Expression
ax9	⊢ (𝑥 = 𝑦 → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))

Proof of Theorem ax9

Dummy variable 𝑡 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		equvinv 2062	. 2 ⊢ (𝑥 = 𝑦 ↔ ∃𝑡(𝑡 = 𝑥 ∧ 𝑡 = 𝑦))
2		ax9v2 2158	. . . . 5 ⊢ (𝑥 = 𝑡 → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑡))
3	2	equcoms 2053	. . . 4 ⊢ (𝑡 = 𝑥 → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑡))
4		ax9v1 2157	. . . 4 ⊢ (𝑡 = 𝑦 → (𝑧 ∈ 𝑡 → 𝑧 ∈ 𝑦))
5	3, 4	sylan9 517	. . 3 ⊢ ((𝑡 = 𝑥 ∧ 𝑡 = 𝑦) → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))
6	5	exlimiv 1963	. 2 ⊢ (∃𝑡(𝑡 = 𝑥 ∧ 𝑡 = 𝑦) → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))
7	1, 6	sylbi 220	1 ⊢ (𝑥 = 𝑦 → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))

This proof depends on syntax axioms:   → wi 4   ∧ wa 401  ∃wex 1812

This proof depends on axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1828  ax-4 1842  ax-5 1943  ax-6 2000  ax-7 2041  ax-9 2155

This proof depends on definitions:  df-bi 210  df-an 402  df-ex 1813

This theorem is used by:  elequ2  2160  elALT2  5331  fv3  6895  elirrvOLDOLD  9577  in-ax8  36983  ss-ax8  36984  bj-ax89  37548  axc11next  45349