Counting

Abstract

Suppose S and T are any two sets, and you want to list all members of S ∩ T. If you list all members of S, then list all the members of T, you will cover all members of S ∩ T, but those in S D T will be listed twice. To count all members of S ∩ T, you could count all members of both lists, then subtract the number of duplicates. In other words,

$$\left| {S \cup T} \right| = \left| S \right| + \left| T \right| - \left| {s \cap T} \right|$$

((2.1))