Theorem 9p9e18

Description: 9 + 9 = 18. (Contributed by Mario Carneiro, 19-Apr-2015)

	Ref	Expression
Assertion	9p9e18	⊢ ( 9 + 9 ) = ; 1 8

Proof

Step	Hyp	Ref	Expression
1		9nn0	⊢ 9 ∈ ℕ0
2		8nn0	⊢ 8 ∈ ℕ0
3		7nn0	⊢ 7 ∈ ℕ0
4		df-9	⊢ 9 = ( 8 + 1 )
5		df-8	⊢ 8 = ( 7 + 1 )
6		9p8e17	⊢ ( 9 + 8 ) = ; 1 7
7	1 2 3 4 5 6	6p5lem	⊢ ( 9 + 9 ) = ; 1 8