\[\begin{array}{l}
+ )\,\,{a^2} + {b^2}\,\,\, \vdots \,\,\,ab\\
\Rightarrow {a^2} + {b^2}\,\,\, \vdots \,\,\,a\\
Ma\,\,\,{a^2}\,\,\, \vdots \,\,\,a \Rightarrow {b^2}\,\, \vdots \,\,\,a\\
\Rightarrow b\,\,\, \vdots \,\,\,a\,\,\,\left( 1 \right).\\
+ )\,\,\,{a^2} + {b^2}\,\,\, \vdots \,\,\,ab\\
\Rightarrow {a^2} + {b^2}\,\,\, \vdots \,\,\,b\\
Ma\,\,\,{b^2}\,\,\, \vdots \,\,\,b \Rightarrow {a^2}\,\, \vdots \,\,\,b\\
\Rightarrow a\,\,\, \vdots \,\,\,b\,\,\,\left( 2 \right).\\
Tu\,\,\,\left( 1 \right)\,\,\,va\,\,\,\left( 2 \right)\\
\Rightarrow a\,\,\,\, \vdots \,\,\,b;\,\,b\,\,\,\, \vdots \,\,\,a\\
\Rightarrow a = b.\\
\Rightarrow A = \frac{{{a^2} + {b^2}}}{{2ab}} = \frac{{{a^2} + {a^2}}}{{2a.a}} = \frac{{2{a^2}}}{{2{a^2}}} = 1.
\end{array}\]
