Consider the following statement. Assume that all sets are subsets of a universal set U. For all sets A and B, if A C B then BC S AC. Use an element argument to construct a proof for the statement by putting selected sentences from the following scrambled list in the correct order. If x were in A, then x would have to be in B by definition of subset. But x € B, and so x € A. Therefore, by definition of complement x E AC, and thus, by definition of subset, BC C AC. By definition of complement, x € B. Suppose A and B are any sets such that A C B, and suppose x E BC. Hence, x € A, because A N B = Ø. Suppose A and B are any sets such that A S B, and suppose x E B.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
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### Understanding Set Theory: Proof involving Complements and Subsets

**Statement:**
Consider the following statement. Assume that all sets are subsets of a universal set \( U \).

For all sets \( A \) and \( B \), if \( A \subseteq B \) then \( B^C \subseteq A^C \).

**Objective:**
Use an element argument to construct a proof for the statement by putting selected sentences from the following scrambled list in the correct order.

**Proof Steps:**

1. **Assumption and Goal:**
   - Suppose \( A \) and \( B \) are any sets such that \( A \subseteq B \), and suppose \( x \in B^C \).

2. **Complement Definition:**
   - By definition of complement, \( x \notin B \).

3. **Subset Implication:**
   - If \( x \) were in \( A \), then \( x \) would have to be in \( B \) by definition of subset. But \( x \notin B \), and so \( x \notin A \).

4. **Complement Membership:**
   - Therefore, by definition of complement \( x \in A^C \), and thus, by definition of subset, \( B^C \subseteq A^C \).

5. **Conclusion:**
   - Hence, \( x \notin A \), because \( A \cap B = \emptyset \).

This proof demonstrates that if one set \( A \) is a subset of another set \( B \), then the complement of \( B \) is a subset of the complement of \( A \), using the concept of set complements and the relationships between subsets.
Transcribed Image Text:### Understanding Set Theory: Proof involving Complements and Subsets **Statement:** Consider the following statement. Assume that all sets are subsets of a universal set \( U \). For all sets \( A \) and \( B \), if \( A \subseteq B \) then \( B^C \subseteq A^C \). **Objective:** Use an element argument to construct a proof for the statement by putting selected sentences from the following scrambled list in the correct order. **Proof Steps:** 1. **Assumption and Goal:** - Suppose \( A \) and \( B \) are any sets such that \( A \subseteq B \), and suppose \( x \in B^C \). 2. **Complement Definition:** - By definition of complement, \( x \notin B \). 3. **Subset Implication:** - If \( x \) were in \( A \), then \( x \) would have to be in \( B \) by definition of subset. But \( x \notin B \), and so \( x \notin A \). 4. **Complement Membership:** - Therefore, by definition of complement \( x \in A^C \), and thus, by definition of subset, \( B^C \subseteq A^C \). 5. **Conclusion:** - Hence, \( x \notin A \), because \( A \cap B = \emptyset \). This proof demonstrates that if one set \( A \) is a subset of another set \( B \), then the complement of \( B \) is a subset of the complement of \( A \), using the concept of set complements and the relationships between subsets.
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