1.
$ C = \left \{ 0; 3; 6; 9; 12; 15 \right \} $
2.
$ A = \left \{ x \in \mathbb{R}/-2 \leq x \leq 3 \right \} = [-2;3] \\ A \cap B = [-2;3] \\ A \cup B = [-1;5] \\ A \setminus B = \varnothing \\ D \cup (A \cap B = D \cup [-2;3] = [-2;5] \\ C \cap (A \cup B = C \cap [-1;5] = [-1;4) \\ R \setminus (C \cup D = R \setminus [-4;5] = (-\infty;4) \cup (5;+\infty ) $
3.
$ \\ A = \left \{ x \in \mathbb{N}/ 11 - 3x > 0 \right \} \\ 11 - 3x > 0 \\ \Leftrightarrow -3x > -11 \\ \Leftrightarrow x < \frac{11}{3} \\ \Rightarrow A = \left \{ 0;1;2;3 \right \} \\ B = \left \{ x \in \mathbb{Z}/|x| - 1 \leq 0 \right \} \\ |x| - 1 \leq 0 \\ \Leftrightarrow |x| \leq 1 \\ \Leftrightarrow -1 \leq x \leq 1 \\ \Rightarrow B = \left \{ -1;0;1 \right \} \\ (A \setminus B) \cup (A \cap B) = \left \{2;3 \right \} \cup \left \{0;1 \right \} = \left \{ 0;1;2;3 \right \} = A $
4.
https://diendan.hocmai.vn/threads/toan.637291/#post-3229779