3.8 Unions and Intersections of Sets

Set operations Let U = {x|x is an English-language film} Set A below contains the five best films according to the American Film Institute. Set B below contains the five best films according to TV Guide. Set C below contains the five most passionate films according to the American Film Institute.

Set operations A = {Citizen Kane, Casablanca, The Godfather, Gone With the Wind, Lawrence of Arabia} B= {Casablanca, The Godfather Part 2, The Wizard of Oz, Citizen Kane, To Kill A Mockingbird} C = {Gone With the Wind, Casablanca, West Side Story, An Affair To Remember, Roman Holiday}.

Set operations Form a new set whose elements are those that sets A and B have in common A = {Citizen Kane, Casablanca, The Godfather, Gone With the Wind, Lawrence of Arabia} B= {Casablanca, The Godfather Part 2, The Wizard of Oz, Citizen Kane, To Kill A Mockingbird} { Citizen Kane, Casablanca} This set is called the INTERSECTION of A and B, denoted A  B. A  B = { Citizen Kane, Casablanca}

Set operations Find B  C B= {Casablanca, The Godfather Part 2, The Wizard of Oz, Citizen Kane, To Kill A Mockingbird} C = {Gone With the Wind, Casablanca, West Side Story, An Affair To Remember, Roman Holiday} B  C = {Casablanca}

Set operations A different operation: form a new set that contains all the elements of A along with all the elements of B. A = {Citizen Kane, Casablanca, The Godfather, Gone With the Wind, Lawrence of Arabia} B= {Casablanca, The Godfather Part 2, The Wizard of Oz, Citizen Kane, To Kill A Mockingbird} {Citizen Kane, Casablanca, The Godfather, Gone With the Wind, Lawrence of Arabia, The Godfather Part 2, The Wizard of Oz, To Kill A Mockingbird} This set is called the union of A with B, denoted A  B.

Set operations We have encountered three basic set operations (including something from Part 1 Module 1). Intersection S  T = {x|x  S and x  T} Union S  T = {x|x  S or x  T}. Complement S = {x|x  S}.

Venn Diagrams A Venn diagram is a drawing in which sets are represented by geometric figures such as circles and rectangles. Venn diagrams can be used to illustrate the relationships between sets, and the effects of set operations. Venn diagrams are also used in other areas of mathematics, such as counting, probability and logic.

Venn Diagrams - Intersection Let S, T represent any sets in a universe U. The Venn diagram below illustrates the effect of intersection. The shaded region corresponds to S  T.

Venn Diagrams - Union Let S, T represent any sets in a universe U. The Venn diagram below illustrates the effect of union. The shaded region corresponds to S  T.

Let S, T represent any sets in a universe U. The Venn diagram below illustrates the effect of complement. The shaded region corresponds to S. Venn diagrams - Complement

Exercise #1 U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} S = {3, 5, 8, 11} T = {3, 4, 6, 7, 8, 10, 11} V = {2, 5, 6, 7, 8} W = {1, 3, 5, 6} Find ( V  S )  ( W  T ) A. { } B. {4, 10, 11} C. {1, 4, 10} D. None of these

Solution #1 U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} S = {3, 5, 8, 11}T = {3, 4, 6, 7, 8, 10, 11} V = {2, 5, 6, 7, 8}W = {1, 3, 5, 6} Find ( V  S )  ( W  T ) First, find V and S V = {1, 3, 4, 9, 10, 11} S = {1, 2, 4, 6, 7, 9, 10} So V  S = {1, 4, 9, 10} Next, T = {3, 4, 6, 7, 8, 10, 11} and W = {1, 3, 5, 6} W  T = {1, 3, 4, 5, 6, 7, 8, 10, 11} Finally, ( V  S )  ( W  T ) = {1, 4, 10}

Exercise #2 U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} T = {3, 4, 6, 7, 8, 10, 11} V = {2, 5, 6, 7, 8} Find (V  T) A. {1, 3, 4, 6, 7, 8, 9, 10, 11} B. {3, 4, 10, 11} C. {1, 3, 4, 6, 8, 9, 10, 11} D. None of these

Solution #2 U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} T = {3, 4, 6, 7, 8, 10, 11} V = {2, 5, 6, 7, 8} Find (V  T) T = {1, 2, 5, 9} and V = {2, 5, 6, 7, 8} so V  T = {2, 5} Since V  T = {2, 5} and U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} (V  T) = {1, 3, 4, 6, 7, 8, 9, 10, 11}

EXAMPLE Venn diagrams From your text: EXAMPLE #13 On the Venn diagram below, shade the region corresponding to A  B.

Solution To shade the region corresponding to A  B, we need to understand the meanings of both terms, and understand the operation of union.

Solution, page 2 Now, apply the inclusive operation of union to the shaded figures for A and B The shaded figure for A  B will include all the shading from the A figure along with all the shading from the B figure.

Exercise #3 Select the Venn diagram whose shaded region corresponds to A  B.

Solution #3 For A  B,we need to draw the shaded figure for A, the shaded figure for B, and then perform the union of the shading from those two figures.

Exercise #4 Select the Venn diagram whose shaded region corresponds to (A  B).

Solution #4 To make the shaded figure (A  B), we need to first make the shaded figure for A  B, and then apply the idea of complement.