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7: Ta có: \(25\cdot\left(-\dfrac{1}{5}\right)^3-2\cdot\left(-\dfrac{1}{2}\right)^2+\dfrac{2}{5}-\dfrac{1}{2}\)
\(=25\cdot\dfrac{-1}{125}-2\cdot\dfrac{1}{4}+\dfrac{2}{5}-\dfrac{1}{2}\)
\(=\dfrac{-1}{5}+\dfrac{2}{5}-\dfrac{1}{2}-\dfrac{1}{2}\)
\(=\dfrac{1}{5}-1=-\dfrac{4}{5}\)
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