gọi  phân thức đối của \(\dfrac{x-1}{x-y}\) là \(A\)

\(A+\dfrac{x-1}{x-y}=0\)

\(\Rightarrow A=0-\dfrac{x-1}{x-y}\)

\(\Rightarrow A=\dfrac{-\left(x-1\right)}{x-y}=\dfrac{1-x}{x-y}\)

\(\Rightarrow ChonC\)

\(\dfrac{x^2-x}{2\times\left(x-1\right)}=\dfrac{x\times\left(x-1\right)}{2\times\left(x-1\right)}=\dfrac{x}{2}\)

\(\Rightarrow ChonB\)