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a)

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thầy nhầm dấu ạ

\(b.Thay\) \(x=2\) \(và\) \(y=-1\) \(vào\) \(B.\) \(Ta\) \(có:\)

\(B\Leftrightarrow\dfrac{1}{2}+\dfrac{1}{-1}\)

\(B=\dfrac{1}{2}-1\)\(=-\dfrac{1}{2}\)

\(a.B=\dfrac{x}{y}.\sqrt{\dfrac{y^2}{x^4}}+\dfrac{1}{x^2}.\sqrt{\dfrac{x^4}{y^2}}\)

   \(B=\dfrac{x}{y}.\dfrac{y}{x^2}+\dfrac{1}{x^2}.\dfrac{x^2}{y}\)

   \(B=\dfrac{1}{x}+\dfrac{1}{y}\)

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