Union and Intersection

Almost every Mathematics test has at least one problem involving Unions and Intersections.

The Union of two sets is the set of elements which are in either set.

Example: A = {1,2,3}, B = {3,4,5}. The UNION of A and B, written AU B = {1, 2, 3, 4, 5}. Notice how none of the numbers are repeated in the final set. Though there is a 3 in set A and a 3 in set B, there is only one 3 in set A U B.


The Intersection of two sets is the set of elements which are in all of the selected sets.
Example: A = {3,5,9}, B = {5, 9, 11}. The INTERSECTION of A and B, written A ∩ B = {5, 9}. Again, multiple numbers are not repeated.