For any two sets A and B, prove that :
A' - B' = B- A
For any two sets A and B , prove that :
A∩B=ϕ⇒A⊆B′.
For any two sets of A and B, prove that :
(i) A′∪B=U⇒A⊂B
(ii) B′⊂A′⇒A⊂B