For any two sets A and B, prove that
A – (A – B) = A ∩ B
Let x ϵ A–(A–B) ⬄ x ϵ A and x ∉ (A–B)
⬄ x ϵ A and x ∉ (A ∩ B)
⬄ x ϵ A∩(A ∩ B)
⬄ x ϵ (A ∩ B)
∴ A–(A–B) = (A ∩ B)