\[\boxed{\text{541\ (541).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[Пусть\ x - первая\ цифра,\ \]
\[а\ y - вторая.\]
\[Составим\ систему\ уравнений:\]
\[\left\{ \begin{matrix} 10x + y = 4 \cdot (x + y) \\ 10x + y = 2xy\ \ \ \ \ \ \ \ \ \ \ \ \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} 6x = 3y\ \ \ \ \ \ \ \ \ \ \ \\ 10x + y = 2xy \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} y = 2x\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ 10x + 2x = 4x^{2} \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} y = 2x\ \ \ \ \ \ \ \ \ \\ x^{2} - 3x = 0 \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} y = 2x\ \ \ \ \ \ \ \ \ \ \ \\ x(x - 3) = 0 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x_{1} = 3 \\ y_{1} = 6 \\ \end{matrix} \right.\ \text{\ \ \ }\]
\[или\ \ \left\{ \begin{matrix} x_{2} = 0 \\ y_{2} = 0 \\ \end{matrix} \right.\ ,\]
\[но\ это\ не\ удовлетворяет\ \]
\[условию\ задачи.\]
\[Ответ:36.\]
\[\boxed{\text{541.}\text{\ }\text{ОК\ ГДЗ\ -\ домашка\ на\ 5}}\]
\[\textbf{а)}\ a_{n} = - 10 + 2n\]
\[a_{1} = - 10 + 2 = - 8;\]
\[a_{2} = - 10 + 4 = - 6;\]
\[d = 2.\]
\[\textbf{б)}\ a_{n} = 8 - 3n\]
\[a_{1} = 8 - 3 = 5;\]
\[a_{2} = 8 - 6 = 2.\]
\[d = - 3.\]