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Exercise Set II
1. Let A and B be subsets of a universe U. Please prove the second De Morgan’s law:
(A ∩ B)c = Ac ∪ Bc
2. Prove that if A, B and C are sets, and if A ⊂ B and B ⊂ C, then A ⊂ C.
c
3. If U := [0,10], A := [3,7) and B := {3,6,9}, then what areand BU ?
4. Let A and B be sets. Please prove or disprove:
P(A ∪ B) = P(A) ∪ P(B)
Hint: Counterexample 5. Prove that for each n ∈ Z+,
1. 2.

6. Please find two distinct proofs that for any n ∈ Z+, then 6 divides n3 − n, that is, 6|(n3 − n).
c
7. Suppose A and B are sets with A ⊂ B. Given the standard definition of AB, use the axioms to show that this complement exists.
8. In terms of axiomatic set theory, please explain why a “set” containing all sets is nota set.
9. Is ∅ the same as {∅}? Explain why or why not. Hint: Cardinality.
10. Please construct on the basis of the axioms a set containing exactly three elements.