f: Ta có: \(\dfrac{4x^2-4xy+y^2}{y^2-4x^2}=\dfrac{A}{2x+y}\)
\(\Leftrightarrow\dfrac{A}{2x+y}=\dfrac{\left(2x-y\right)^2}{-\left(2x-y\right)\left(2x+y\right)}\)
\(\Leftrightarrow A=\dfrac{-\left(2x-y\right)^2\cdot\left(2x+y\right)}{\left(2x-y\right)\left(2x+y\right)}=-2x+y\)
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