a: ĐKXĐ: x<>0
\(\dfrac{x+1}{6x}-\dfrac{3x-1}{6x}\)
\(=\dfrac{x+1-3x+1}{6x}\)
\(=\dfrac{-2x+2}{6x}=\dfrac{-x+1}{3}\)
b: ĐKXĐ: \(x\notin\left\{4;1\right\}\)
\(\dfrac{2x}{4-x}:\dfrac{2}{x-1}\)
\(=\dfrac{-2x}{x-4}\cdot\dfrac{x-1}{2}\)
\(=\dfrac{-x\left(x-1\right)}{x-4}\)