Theorem 9p9e18

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

	Ref	Expression
Assertion	9p9e18	$$⊢9+9=18$$

Proof

Step	Hyp	Ref	Expression
1		9nn0	$$⊢9\in {\mathbb{N}}_0$$
2		8nn0	$$⊢8\in {\mathbb{N}}_0$$
3		7nn0	$$⊢7\in {\mathbb{N}}_0$$
4		df-9	$$⊢9=8+1$$
5		df-8	$$⊢8=7+1$$
6		9p8e17	$$⊢9+8=17$$
7	1 2 3 4 5 6	6p5lem	$$⊢9+9=18$$