For any two sets A and B, prove the following:
A ∩ (A ∪ B’) = ϕ
= A∩ (A ∪ B’)
= A∩( A’∩ B’) [By De–morgan’s law]
= (A ∩ A’) ∩ B’ [∴ A ∩ A’ = ϕ ]
= ϕ ∩ B’
= ϕ
=RHS