Question

For all sets A, B, and C

Is (A – B) ∩ (C – B) = (A ∩ C) – B? Justify your answer.

Answer 1

Yes

Let x ∈ (A – B) ∩ (C – B)

⇒ x ∈ A – B and x ∈ C – B

⇒ (x ∈ A and x ∉ B) and (x ∈ C and x ∉ B)

⇒ (x ∈ A and x ∈ C) and x ∉ B

⇒ (x ∈ A ∩ C) and x ∉ B

⇒ x ∈ (A ∩ C) – B

So (A – B) ∩ (C – B) ⊂ (A ∩ C) – B   ......(1)

Now, conversely

Let y ∈ (A ∩ C) – B

⇒ y ∈ (A ∩ C) and y ∉ B

⇒ (y ∈ A and y ∈ C) and (y ∉ B)

⇒ (y ∈ A and y ∉ B) and (y ∈ C and y ∉ B)

⇒ y ∈ (A – B) and y ∈ (C – B)

⇒ y ∈ (A – B) ∩ (C – B)

So (A ∩ C) – B ⊂ (A – B) ∩ (C – B)  ......(2)

From (1) and (2),

(A – B) ∩ (C – B) = (A ∩ C) – B