1. Warning: All the Venn Diagram construction and pictures will be done during class and are not included in this presentation. If you missed class you should get class notes from another student.
Sets Day 2

2. Venn Diagrams
Example. Let U be the set of natural numbers less than or equal to 10. Let A={2,4,6} and B={1,2,3,4,5}. (Note: I constructed this Venn Diagram during class and referred to it throughout this presentation.)

3. Definition of Set Complement
The complement of set A, symbolized by A′, is the set of all the elements in the universal set that are not in the set A. A′ is read “A complement,” or “A prime.”

4. Set Complement
Example. Let U be the set of natural numbers less than or equal to 10. Let A={2,4,6} and B={1,2,3,4,5}. Find A′ and B′.
Answers:
A′ = {1,3,5,7,8,9,10}
B′ = {6,7,8,9,10}

5. Definition of Set Intersection
The intersection of sets A and B, symbolized by A∩B, is the set containing all the elements that are common to both A and B. A∩B is read “A intersect B,” or “A AND B.”

6. Set Intersection
Example. Let U be the set of natural numbers less than or equal to 10. Let A={2,4,6} and B={1,2,3,4,5}. Find A∩B. Answer: A∩B = {2,4}

7. Example
Let U = {a, b, c, d} A = {a, c} B = {b, d} 1. Find A′. 2. Find U′. 3. Find φ′. 4. Find A∩B. 5. Find (A∩B)′.
Answers:
A′ = B
U′ = φ
φ′ = U
A∩B = φ
(A∩B)′= U

8. Definition of Set Union
The union of set A and set B, symbolized by A∪B, is the set containing all the elements that are members of set A or of set B (or of both sets). A∪B is read “A union B,” or “A OR B.”

9. Set Union
Example. Let U be the set of natural numbers less than or equal to 10. Let A={2,4,6} and B={1,2,3,4,5}. Find A∪B. Answer: A∪B = {1,2,3,4,5,6}

10. Example
Let U = {a, b, c, d} A = {a, c} B = {b, d} 1. Find A∪B. 2. Find (A∪B )′. 3. Find A′∩B. 4. Find (A′∩B)′.
Answers:
A∪B = U
(A∪B )′ = φ
A′∩B = B
(A′∩B)′ = A

11. Union Rule for Sets n(A∪B) = n(A) + n(B) – n(A∩B)
The relationship between sets A, B, A∪B, and A∩B is given by the union rule:
n(A∪B) = n(A) + n(B) – n(A∩B)
Why? (We used aVenn Diagram to show this.)

12. Two Possible Union Rule Test Problems:
If n(A) = 5, n(B) = 8, and n(A∩B) = 2, find n(A∪B).
Answer: n(A∪B)=5+8-2=11
If n(A)=12, n(A∪B)=22, and n(A∩B)=10, find n(B).
Answer: 20

13. Difference of Two Sets
The difference of two sets A and B, symbolized A-B, is the set of elements that belong to set A but not to set B. Example. Let U ={1,2,3,…,10}, A={2,4,6} and B={1,2,3,4,5}. Find A-B and B-A. A-B={6} B-A={1,3,5}