##<b>Basic set notation.</b> Let $A$ and $B$ be sets. #$x\in A$ means $x$ is an element of $A$ #$x\notin A$ means $x$ is not an element of $A$ #$\{x\in A\ |\ P(x) \}=$ the set of all $x\in A$ such that $P(x)$ is true (this is called set-builder notation; $P(x)$ is any property that is either true or false depending on $x$) #$A=B$ is true when $A$ and $B$ have the same elements #$A \cup B=$ $\{x \ |\ x \in A \mbox{or} x \in B \mbox{ (or both)}\}$ #$A \cap B=$ $\{x \ |\ x \in A \mbox{and} x \in B\}$ <br>