5 7 11 is a set. That is the set of all elements in U the universal set for A that are not in A. We use special notation to indicate whether or not an element belongs to a set as shown below.
For example 14 Q and 116 Q.
A the area shaded in blue b the area shaded in purple. An object that belongs to a set is called an element or a member of that set. Look at the venn diagram on the left. A Set is a collection of things usually numbers.