Find the mistake in the following “proof.”“Theorem:” For

Chapter 6, Problem 21E

(choose chapter or problem)

Problem 21E

Find the mistake in the following “proof.”

Theorem:” For all sets A and B, Ac ∪ Bc ⊆ (A ∪ B)c.

Proof: Suppose A and B are sets, and x ∈ Ac ∪ Bc. Then x ∈ Ac or x ∈ Bc by definition of union. It follows that x ∈ A or x ∈ B by definition of complement, and so x ∈ A ∪ B by definition of union. Thus x ∈ (A ∪ B )c by definition of complement, and hence Ac ∪ Bc ⊆ (A ∪ B)c.”

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