Theorem 2.1: (Commutativity of Unions) Let A and B be two sets. i ) The complement is … For example, we can start by using one of the basic properties in Theorem 5.20 to write, We can then use one of the commutative properties to write, \[\begin{array} {rcl} {(A \cup B) - C} &= & {(A \cup B) \cap C^c} \\ {} &= & {C^c \cap (A \cup B).} The intersection is notated A ⋂ B.. More formally, x ∊ A ⋂ B if x ∊ A and x ∊ B Theorem Given a set } The intersection of two sets \(A\) and \(B\), denoted \(A\cap B\), is the set of elements common to both \(A\) and \(B\). → ∃ , \end{array}\], 13. holds is denoted {\displaystyle \bigcap S=\{2,3\}} 5 , joined by the logical conjunction and from formal logic. we denote C S { x Similarly, Union, Intersection, and Complement. x } Also notice how nicely a proof dealing with the union of two sets can be broken into cases. x x ⋃ S The construction that allows us to form sets with more than two elements is the union. Based on the Venn diagrams in Part (1), what appears to be the relationship between the sets ((A \cup B)^c\) and \(A^c \cap B^c\)? Remember that \(X \leftrightarrow Y\) is logically equivalent to \((X \to Y) \wedge (Y \to X)\). ∈ AssumeP(A)µP(B). {\displaystyle S} S Prove or disprove each of the following. Similarly Thus, \(y \in B \cap C\) and, hence, \(y \in A \cup (B \cap C)\). ∈ is an existential quantifier. Proof. {\displaystyle A=\{1,2,3\}} {\displaystyle P(x,A)} and {\displaystyle B=\{1,2\}} , 2 , ( the expression Complete a truth table for. for some , i Two relationships in the next theorem are known as De Morgan’s Laws for sets and are closely related to De Morgan’s Laws for statements. For any two finite sets A and B; (i) (A U B)' = A' ∩ B' (which is a De Morgan's law of union). x \[\begin{array} {rcl} {(A \cup B) - C} &= & {(A \cup B) \cap C^c \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(Theorem 5.20)}} \\ {} &= & {C^c \cap (A \cup B) \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(Commutative Property)}} \\ {} &= & {(C^c \cap A) \cup (C^c \cap B)} \\ {} &= & {(A \cap C^c) \cup (B \cap C^c)} \\ {} &= & {(A - C) \cup (B - C)} \end{array}\]. {\displaystyle x\in S_{1}} This page was last edited on 6 June 2017, at 20:41. {\displaystyle A\cup B} A Theorem 2.2: (Commutativity of Intersections) Let A and B be two sets. ∈ and, hence, we have proved that \(A \cap B = B \cap A\). . If anybody knows where I can find examples of these types of proofs, it would be greatly appreciated.Can I get a proof verification? = ∃ B Proof By the Axiom of Pair, {\displaystyle A\in S} x {\displaystyle A\subseteq B} = is called a universal quantifier. ) 3 1 We now use the fact that \(A = A \cap A\) and obtain, \[\begin{array} {rcl} {A - (B \cup C)} &= & {A \cap A \cap B^c \cap C^c} \\ {} &= & {(A \cap B^c) \cap (A \cap C^c)} \\ {} &= & {(A - B) \cap (A - C).} ) ∩ A Memorize the definitions of intersection, union, and set difference. , = of sets, we call a set 2 → } . But \(x \in B\) implies that \(x \in A \cup B\), and \(x \in C\) implies that \(x \in A \cup C\). } A For example, one of the distributive properties from Theorems 5.18 can be written as follows: For all sets \(X\), \(Y\), and \(Z\) that are subsets of a universal set \(U\), \((X \cap (Y \cup Z) = (X \cap Y) \cup (X \cap Z).\). A Consequently, we can conclude that, \[x \in A \text{ and } x \in B \text{ if and only if } x \in B \text{ and } x \in A.\], \[x \in B \text{ and } x \in A \text{ if and only if } x \in B \cap A.\], This means that we can use (5.3.1), (5.3.2) and (5.3.3) to conclude that. A useful way to remember the symbol is ∪ \cup ∪ nion. are disjoint since their intersection is empty. {\displaystyle \exists !} A . S P For example, A holds. for some Use one of De Morgan’s Laws (Theorem 2.8 on page 48) to explain carefully what it means to say that an element \(x\) is not in \(A \cup B\). x So we consider the case that \(y \notin A\). Explain carefully how you determined these regions. Comprehensions allow us to select elements of an existing set that have some specified property. . We rely on them to prove or derive new results. {\displaystyle \bigcup S=\{1,2,3,5\}} {\displaystyle B} P iff Explain carefully what it means to say that an element \(x\) is in \(A^c \cap B^c\). Legal. {\displaystyle \{x\in A\;|\;P(x,A)\}} ∈ Toshow AµB,supposethata2. Let \(A\) be a subset of some universal set \(U\). Additive Identity: For all \(a \in \mathbb{R}\), \(a + 0 = a = 0 + a\). {\displaystyle S_{1}} Let \(A\), \(B\), and \(C\) be subsets of some universal set \(U\). 2 ◻ IfP (A )µP B,then A µB. ◻ {\displaystyle x\in S_{2}} {\displaystyle x\in S_{1}} {\displaystyle S} = is a set. for which a property {\displaystyle A\cup B=\bigcup S} Definition If \((A \cup B) \cup C = A \cup (B \cup C)\), Distributive Laws \(A \cap (B \cup C) = (A \cap B) \cup (A \cap C)\) as in the Axiom of Union, a union over 3 S From Wikibooks, open books for an open world, Mathematical Proof and the Principles of Mathematics, Mathematical Proof and the Principles of Mathematics/Sets, https://en.wikibooks.org/w/index.php?title=Mathematical_Proof_and_the_Principles_of_Mathematics/Sets/Union_and_intersection&oldid=3226813, Book:Mathematical Proof and the Principles of Mathematics. 2 So in one case, if \(x \in A\), then \(x \in A \cup B\) and \(x \in A \cup C\). }

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