A set is a collection of items called elements. We write $A = \{2, 4, 6, 8\}$, and $n(A) = 4$ is the number of elements.
The symbol $\in$ means "is an element of". The intersection $A \cap B$ is what is in both sets; the union $A \cup B$ is everything in either set. A set with no elements is the empty set.
Remember
- $n(A)$ = how many elements are in set $A$.
- $\cap$ = in both · $\cup$ = in either.