For any two sets A and B, prove that
A ⊂ B A ∩ B = A
Let
p ϵ A ⊂ B.
⇒ x ϵ B
Let and p ϵ A ∩ B
⬄ x ϵ A and x ϵ B
∴ (A ∩ B) = A.