a: \(=\dfrac{4x+13+x-48}{5x\left(x-7\right)}=\dfrac{5x-35}{5x\left(x-7\right)}=\dfrac{5\left(x-7\right)}{5x\left(x-7\right)}=\dfrac{1}{x}\)
b: \(=\dfrac{-1}{5x^2-x}-\dfrac{25x-15}{25x^2-1}\)
\(=\dfrac{-1}{x\left(5x-1\right)}-\dfrac{25x-15}{\left(5x-1\right)\left(5x+1\right)}\)
\(=\dfrac{-5x-1-25x^2+15x}{x\left(5x-1\right)\left(5x+1\right)}\)
\(=\dfrac{-25x^2+10x-1}{x\left(5x-1\right)\left(5x+1\right)}=\dfrac{-\left(5x-1\right)}{x\left(5x+1\right)}\)
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