In this section we will learn about intersection and union of 2 sets.

Let A and B be sets. Then, the definitions for intersection and union is the following:

** Intersection**: A set of elements where the elements show up both in A and B. We call this intersection A$\cap$B. Sometimes people refer to the symbol $\cap$ as the word "and".

__ Union__: A set of all elements that appears in A, in B, or both in A and B. We call this union A$\cup$B. Sometimes people refer to the symbol $\cup$ as the word "or".

Here is a definition that may be useful:

A\B: The set of elements that is in A but **not** in B. In short, it is just A minus B.

The principle of inclusion and exclusion of 2 sets says the following: