Compound Inequalities

Union and Intersection of Sets

Union of Sets

The union of two sets, denoted AB ("A union B"), is the set of all members contained in either A or B or both. We can think of the union of two sets as the entire Venn diagram: Union of Sets

In our first example, AB = {0, 1, 2, 3, 4, 5, 6}: Union of {1, 2, 3, 4, 5, 6} and {0, 2, 4, 6}
In our second example, AB = { -10, -2, 0, , 4, 11, 15}: Union of A and B
Example 3: What is XY if X = { -4, 3, 2, 11, -6} and Y = {3, 6, 11, -4, 5}?

The easiest way to write the union of two sets is to write all the members in the first set, and then write all the members in the second set that haven't been written yet:
XY = { -4, 3, 2, 11, -6, 6, 5}

Example 4: What is AB if A = {2, 3, 4} and B = {5, 6, 7}?

AB = {2, 3, 4, 5, 6, 7}

The number of members in AB should be the total number of members in A plus the total number of members in B minus the number of members which are in both sets. In example 3, X contains 5 members, Y contains 5 members, and there are 3 members which are in both X and Y. Thus, XY has 5 + 5 - 3 = 7 members.

Intersection of Sets

The intersection of two sets, denoted AB ("A intersect B") is the set of all members contained in bothA and B. We can think of the intersection of two sets as the overlap in the Venn diagram: Intersection of Sets

In our first example, AB = {2, 4, 6}: Intersection of {1, 2, 3, 4, 5, 6} and {0, 2, 4, 6}
In our second example, AB = : Intersection of A and B
Example 3. What is XY if X = { -4, 3, 2, 11, -6} and Y = {3, 6, 11, -4, 5}?
XY = { -4, 3, 11}
Example 4: What is AB if A = {2, 3, 4} and B = {5, 6, 7}?

Since A and B have no members in common, AB = ò (the null set).