[tex]\overline{x,y}+\overline{y,x}=\dfrac{\overline{xy}}{10}+\dfrac{\overline{yx}}{10}=\dfrac{\overline{xy}+\overline{yx}}{10}=\dfrac{10x+y+10y+x}{10}=\dfrac{11(x+y)}{10}\\
\in \mathbb{N}\Leftrightarrow x+y\in M_{10}\\
Dar\ cum: x+y\ \textless \ 20\Rightarrow x+y=10\\
S:(x,y)\in \{(1,9);(2,8);(3,7);_{\dots};(9,1)\}[/tex]