Theorem ex-1st

Description: Example for df-1st . Example by David A. Wheeler. (Contributed by Mario Carneiro, 18-Jun-2015)

	Ref	Expression
Assertion	ex-1st	⊢ ( 1st ‘ ⟨ 3 , 4 ⟩ ) = 3

Proof

Step	Hyp	Ref	Expression
1		3ex	⊢ 3 ∈ V
2		4re	⊢ 4 ∈ ℝ
3	2	elexi	⊢ 4 ∈ V
4	1 3	op1st	⊢ ( 1st ‘ ⟨ 3 , 4 ⟩ ) = 3