\[\boxed{\mathbf{1.}}\]
\[\textbf{а)}\ 3 < a < 7\]
\[a = \left\{ 4;5;6 \right\}.\]
\[\textbf{б)}\ 3 \leq a < 7\]
\[a = \left\{ 3;4;5;6 \right\}.\]
\[\textbf{в)}\ 3 < a \leq 7\]
\[a = \left\{ 4;5;6;7 \right\}.\]
\[\textbf{г)}\ 3 \leq a \leq 7\]
\[a = \left\{ 3;4;5;6;7 \right\}.\]
\[\boxed{\mathbf{2.}}\]
\[\textbf{а)}\ x \geq 5\]
\[x = \left\{ 5;6;7;\ldots \right\}.\]
\[\textbf{б)}\ 2 < n \leq 6\]
\[n = \left\{ 3;4;5;6 \right\}.\]
\[\boxed{\mathbf{3.}}\]
\[y < 351 + (50\ 006 - 4859)\ :3\]
\[351 + 15\ 049 = 15\ 400\]
\[y < 15\ 400.\]
\[Наибольшее\ решение:\ \]
\[y = 15\ 399.\]
\[Ответ:\ 15\ 399\]
\[\boxed{\mathbf{4.}}\]
\[\textbf{а)}\ (b + b\ :3) \cdot 2\ дм.\]
\[\textbf{б)}\ \ c \cdot (c + 3)\ м^{2}.\]
\[\boxed{\mathbf{5.}}\]
\[A = \left\{ 5;6;7;8 \right\};\ \ \]
\[B = \left\{ 5;6;7;8;9 \right\}.\]
\[A \cap B = \left\{ 5;6;7;8 \right\};\]
\[A \cup B = \left\{ 5;6;7;8;9 \right\}.\]