Theorem 9on

Description: Ordinal 9 is an ordinal number. (Contributed by BTernaryTau, 4-Sep-2026)

	Ref	Expression
Assertion	9on	⊢ 9o ∈ On

Proof

Step	Hyp	Ref	Expression
1		df-9o	⊢ 9o = suc 8o
2		8on	⊢ 8o ∈ On
3	2	onsuci	⊢ suc 8o ∈ On
4	1 3	eqeltri	⊢ 9o ∈ On