Exercise C.2.2

Prove Boole's inequality: For any finite or countably infinte sequence of events $A_1, A_2, \ldots$,

$$ \Pr\{A_1 \cup A_2 \cup \ldots \} \le \Pr\{A_1\} + \Pr\{A_2\} + \ldots $$

$$ \begin{aligned} \Pr\{A_1 \cup A_2 \cup \cdots\} &= \Pr\{A_1\} + \Pr\{A_2 \cup A_3 \cup \cdots\} - \Pr\{A_1 \cap (A_2 \cup A_3 \cup \cdots)\} \\ & \le \Pr\{A_1\} + \Pr\{A_2 \cup A_3 \cup \cdots\} \\ & \le \Pr\{A_1\} + \Pr\{A_2\} + \Pr\{A_3 \cup A_4 \cup \cdots\} \end{aligned} $$

Strictly speaking, we should use some induction, but you get the idea.