For any two sets, prove that:
A ∪ (A ∩ B) = A
A ∪ (A ∩ B) [∵ union is distributive over intersection]
(A ∪ A) ∩ (A ∪ B)[∵ A ∪ A = A]
= A ∩ (A ∪ B)
= A.