\[\boxed{\text{744\ (744).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\overline{\text{\ ab}} + \overline{\text{ba}} =\]
\[= 10a + b + 10b + a =\]
\[= 11a + 11b =\]
\[= 11 \cdot (a + b) \Longrightarrow кратно\ \]
\[(a + b).\]
\[\textbf{б)}\ \overline{\text{ab}} - \overline{\text{ba}} =\]
\[= 10a + b - 10b - a =\]
\[= 9a - 9b =\]
\[= 9 \cdot (a - b) \Longrightarrow кратно\ 9.\]