${A_n} = \left[ {\frac{1}{{n + 1}},\frac{1}{n}} \right]$
الف) $\mathop \cup \limits^4 A_1^0 = {A_1} \cup {A_2} \cup {A_3} \cup {A_4}$
$ \to \left\{ \begin{gathered}
{A_1} = \left[ {\frac{1}{2},1} \right] \hfill \\
{A_2} = \left[ {\frac{1}{3},\frac{1}{2}} \right] \hfill \\
{A_3} = \left[ {\frac{1}{4},\frac{1}{3}} \right] \hfill \\
{A_4} = \left[ {\frac{1}{5},\frac{1}{4}} \right] \hfill \\
\end{gathered} \right. \Rightarrow \mathop \cup \limits_{i = 1}^4 {A_i} = \left[ {\frac{1}{5},1} \right]$
ب) $\mathop \cup \limits_{i = 1}^4 {A_i} = {A_1} \cap {A_2} \cap {A_3} \cap {A_4} = \emptyset $