Đáp án:
$\begin{array}{l}
\left\{ \begin{array}{l}
10x - 9y = 1\\
15x + 21y = 36
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
10x - 9y = 1\\
5x + 7y = 12
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
10x - 9y = 1\\
10x + 14y = 24
\end{array} \right.\\
\Rightarrow 14y + 9y = 24 - 1\\
\Rightarrow 23y = 23\\
\Rightarrow y = 1\\
\Rightarrow 10x = 9y + 1 = 10\\
\Rightarrow x = 1\\
Vậy\,\left( {x;y} \right) = \left( {1;1} \right)
\end{array}$
