$$ Q \div \frac{M}{N} = Q \cdot \frac{N}{M} $$
[tex]A = \frac{(\sqrt{x}+\sqrt{y})^{2}}{(x-y) \div \left(\sqrt{\frac{1}{y}}+3\sqrt{\frac{1}{x}} \right)} \div \frac{x+9y+6\sqrt{xy}}{\frac{1}{\sqrt{x}}+\sqrt{\frac{1}{y}}}[/tex]
[tex]\sqrt{x} = u \Rightarrow \begin{cases} x=u^{2} \\ \sqrt{\frac{\normalsize{1}}{\normalsize{x}}}=\frac{\normalsize{1}}{\sqrt{\normalsize{x}}}=\frac{\normalsize{1}}{\normalsize{u}} \end{cases}; \hspace{3em} \sqrt{y} = v \Rightarrow \begin{cases} y=v^{2} \\ \sqrt{\frac{\normalsize{1}}{\normalsize{y}}}=\frac{\normalsize{1}}{\sqrt{\normalsize{y}}}=\frac{\normalsize{1}}{\normalsize{v}} \end{cases}[/tex]
[tex]\Rightarrow A=\frac{(u+v)^{2}}{(u^{2}-v^{2}) \div \left(\frac{\normalsize{1}}{\normalsize{v}}+3\cdot \frac{\normalsize{1}}{\normalsize{u}} \right)} \div \frac{u^{2}+2\cdot{u}\cdot{3v}+(3v)^{2}}{\frac{\normalsize{1}}{\normalsize{u}}+\frac{\normalsize{1}}{\normalsize{v}}}[/tex]
[tex]\frac{\normalsize{1}}{\normalsize{v}}+3\frac{\normalsize{1}}{\normalsize{u}}=\frac{u+3v}{uv}; \hspace{3em} \frac{\normalsize{1}}{\normalsize{u}}+\frac{\normalsize{1}}{\normalsize{v}}=\frac{u+v}{uv}[/tex]
[tex]A=\frac{(u+v)^{\cancel{2}}}{\cancel{(u+v)}(u-v)\cdot{\frac{\normalsize{uv}}{\normalsize{\cancel{u+3v}}}}}\cdot \frac{\frac{\normalsize{u+v}}{\normalsize{uv}}}{(u+3v)^{\cancel{2}}}=\frac{(u+v)(u+v)}{(u-v)\cdot uv \cdot (u+3v) \cdot uv}[/tex]
[tex]A=\frac{(u+v)^{2}}{u^{2}v^{2}(u-v)(u+3v)} \Rightarrow A=\frac{(\sqrt{x}+\sqrt{y})^{2}}{xy(\sqrt{x}-\sqrt{y})(\sqrt{x}+3\sqrt{y})}=\frac{x+2\sqrt{xy}+y}{xy(x+2\sqrt{xy}-3y)}[/tex]
[tex]\color{lightseagreen}\text{''Който никога не е правил грешка, никога не е опитвал нещо ново.''} \\
\hspace{21em}\text{(Алберт Айнщайн)}[/tex]