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a)
2x^{-4}=\frac{2}{x^4}
\frac{3}{5}x^{-2}=\frac{3}{5x^2}
\left(\frac{5}{4x}\right)^{-2}=\frac{16x^2}{25}
b)
\frac{1}{2x^3}=2x^{-3}
\frac{2}{3x^5}=\frac{2}{3}x^{-5}