\[\boxed{\mathbf{1.}}\]
\[\textbf{а)}\ 2 < a < 5\]
\[a = \left\{ 3;4 \right\}.\]
\[\textbf{б)}\ 2 \leq a < 5\]
\[a = \left\{ 2;3;\ 4 \right\}.\]
\[\textbf{в)}\ 2 < a \leq 5\]
\[a = \left\{ 3;4;5 \right\}.\]
\[\textbf{г)}\ 2 \leq a \leq 5\]
\[a = \left\{ 2;3;4;5 \right\}.\]
\[\boxed{\mathbf{2.}}\]
\[\textbf{а)}\ y \leq 4\]
\[y = \left\{ 0;1;2;3;4 \right\}.\]
\[\textbf{б)}\ 3 < a \leq 7\]
\[n = \left\{ 4;5;6;7 \right\}.\]
\[\boxed{\mathbf{3.}}\]
\[c > 894 + (91\ 004 - 16\ 479)\ :5\]
\[c > 15\ 799.\]
\[Наименьшее\ решение:\ \]
\[c = 15\ 800.\]
\[Ответ:\ 15\ 800.\]
\[\boxed{\mathbf{4.}}\]
\[\textbf{а)}\ P = (n + n \cdot 4) \cdot 2\ см.\]
\[\textbf{б)}\ S = d \cdot (d - 7)\ {см}^{2}.\]
\[\boxed{\mathbf{5.}}\]
\[A = \left\{ 3;4;\ 5;6;7 \right\};\ \ \]
\[B = \left\{ 5;6;7;8;9 \right\}.\]
\[A \cap B = \left\{ 5;6;7 \right\};\]
\[A \cup B = \left\{ 3;4;\ 5;6;7;8;9 \right\}.\]