1. [TEX]x \in A \cap B \Rightarrow x \in A, x\in B \Rightarrow x \in A \Rightarrow (A \cap B) \subset A[/TEX]
2. [TEX]x \in A \setminus B \Rightarrow x \in A, x \notin B \Rightarrow x \in A \Rightarrow (A \setminus B) \subset A[/TEX]
3. [TEX]x \in (A \setminus B) \setminus C \Rightarrow [/TEX][tex]\left\{\begin{matrix} x \in A \setminus B\\ x \notin C \end{matrix}\right.\Rightarrow \left\{\begin{matrix} x \in A\\ x \notin B\\ x \notin C \end{matrix}\right.\Rightarrow x \in A \Rightarrow (A\setminus B)\setminus C \subset A[/tex]
4. [tex]x \in A \cup B \Rightarrow x \in (A \cup B) \cup C \Rightarrow x \in A \cup B \cup C \Rightarrow (A \cup B) \subset (A \cup B \cup C)[/tex]
5. [tex]x \in A \cap B \cap C \Rightarrow \left\{\begin{matrix} x \in A\\ x \in B\\ x \in C \end{matrix}\right.\Rightarrow \left\{\begin{matrix} x \in A\\ x \in B \end{matrix}\right.\Rightarrow x \in A \cap B \Rightarrow (A \cap B \cap C) \subset (A \cap B)[/tex]
6. [TEX]x \in \overline{A \cap B} \Rightarrow x \notin A, x \notin B \Rightarrow x \notin A \Rightarrow x \in \overline{A} \Rightarrow \overline{A \cap B} \subset \overline{A}[/TEX]
Lưu ý: [TEX]A \subset B[/TEX] khi và chỉ khi [TEX]x \in A \Rightarrow x \in B[/TEX]