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b) HPT
$\Leftrightarrow \begin{cases}x^{2} + y^{2} +x-y =4 \\x^{2} + y^{2} +x-y -xy=2 \end{cases}$
$\Leftrightarrow \begin{cases}x^{2} + y^{2} +x-y =4 \\xy=2 \end{cases}$
$\Leftrightarrow \begin{cases}x^{2} + \dfrac{4}{x^2} +x-\dfrac{2}{x} =4 \\y=\dfrac{2}{x} \end{cases}$
$\Leftrightarrow \begin{cases}x^{2} + \dfrac{4}{x^2} +x-\dfrac{2}{x} =4 \\y=\dfrac{2}{x} \end{cases}$
$\Leftrightarrow \begin{cases}(x^{2} -2)(x+2)(x-1) =0 \\y=\dfrac{2}{x} \end{cases}$
Vậy $(x,y)=(1.2),(-2,-1),(\sqrt 2,\sqrt 2),(-\sqrt 2,-\sqrt 2).$
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