Final answer:
The union of sets A and B is {1, 2, 3, 4, 5, 8, 9, 10, 11, 12} with a cardinality of 10.
Step-by-step explanation:
The union of sets A and B, denoted as A ∪ B, contains all the elements that are in either set A or set B (or both). In this case, A = {5, 4, 3, 2, 1} and B = {8, 5, 9, 4, 10, 11, 2, 12}. To find the union, we combine all the unique elements from both sets:
A ∪ B = {1, 2, 3, 4, 5, 8, 9, 10, 11, 12}
The cardinality of a set represents the number of elements it contains. So, |A ∪ B| represents the cardinality of the union of sets A and B, which is the number of elements in the union:
|A ∪ B| = 10