Đáp án:
$\begin{array}{l}
2)\\
+ 25{x^2} + 16{y^2} = 41xy\\
\Rightarrow 25{x^2} + 2.5x.4y + 16{y^2} = 41xy + 40xy\\
\Rightarrow {\left( {5x + 4y} \right)^2} = 81xy\\
\Rightarrow 5x + 4y = - 9\sqrt {xy} \left( {do:4y < 5x < 0} \right)\\
+ 25{x^2} + 16{y^2} = 41xy\\
\Rightarrow 25{x^2} - 2.5x.4y + 16{y^2} = 41xy - 40xy\\
\Rightarrow {\left( {5x - 4y} \right)^2} = xy\\
\Rightarrow 5x - 4y = \sqrt {xy} \\
\Rightarrow M = \frac{{5x - 4y}}{{5x + 4y}} = \frac{{\sqrt {xy} }}{{ - 9\sqrt {xy} }} = - \frac{1}{9}
\end{array}$