[tex]\it a)\ \ x\in\mathbb{N}\ \ \ \ (*)\\ \\ x<\sqret{18} \Rightarrow x<\sqrt{25} \Rightarrow x<5 \stackrel{(*)}{\Longrightarrow}\ x\in\{0,\ 1,\ 2,\ 3,\ 4\} =A\\ \\ \dfrac{6}{x-3}\in\mathbb{N}\Rightarrow x-3|6\ \c{s}i\ x-3>0 \Rightarrow x-3\in\{1,\ 2,\ 3,\ 6\}|_{+3}\Rightarrow\\ \\ \Rightarrow x\in\{4,\ 5,\ 6,\ 9\}=B[/tex]
Mulțimile cerute sunt:
A = {0, 1, 2, 3, 4}, B = {4, 5, 6, 9}
b) A - B = {0, 1, 2, 3}
A ∪ B = {0, 1, 2, 3, 4, 5, 6, 9 }
