a)
[tex]\it 2| \overline{abba} \Rightarrow a\in\{0,\ 2,\ 4,\ 6.\ 8\}\ \ \ \ (1)\\ \\ a+b=13\ \ \ \ (2)\\ \\ (1),\ (2) \Rightarrow \begin{cases}\it a=4,\ b=9 \Rightarrow \overline{abba}=4994\\ \\ \it a=6,\ b=7 \Rightarrow \overline{abba}=6776\\ \\ \it a=8,\ b=5 \Rightarrow \overline{abba}=8558\end{cases}[/tex]
b)
[tex]\it 5|\ \overline{abba} \Rightarrow a=0,\ sau\ a=5\ \ \ \ \ (3)\\ \\ a+b=12\ \ \ \ \(4)\\ \\ (3),\ (4) \Rightarrow a=5,\ b=7,\ \overline{abba}=5775[/tex]
