\(A=\dfrac{3x^2-2xy+3xy-2y^2}{3x^2+3xy+2xy+2y^2}=\dfrac{\left(x+y\right)\left(3x-2y\right)}{\left(x+y\right)\left(3x+2y\right)}=\dfrac{3x-2y}{3x+2y}\)
\(9x^2+4y^2=20xy\\ \Leftrightarrow9x^2-20xy+4y^2=0\\ \Leftrightarrow9x^2-18xy-2xy+4y^2=0\\ \Leftrightarrow\left(x-2y\right)\left(9x-2y\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=2y\\9x=2y\end{matrix}\right.\)
\(\forall x=2y\Leftrightarrow A=\dfrac{6y-2y}{6y+2y}=\dfrac{4y}{8y}=\dfrac{1}{2}\\ \forall9x=2y\Leftrightarrow A=\dfrac{3x-9x}{3x+9x}=\dfrac{-6x}{12x}=-\dfrac{1}{2}\)
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