Ta có : \(A=\left(1-\frac{z}{x}\right)\left(1+\frac{x}{y}\right)\left(1-\frac{y}{z}\right)=\frac{x-z}{x}\cdot\frac{x+y}{y}\cdot\frac{z-y}{z}\)
\(x+y-z=0\Leftrightarrow\hept{\begin{cases}x+y=z\\x-z=-y\\z-y=x\end{cases}}\) thay vào A ta được :
\(A=\frac{-y}{x}\cdot\frac{z}{y}\cdot\frac{x}{z}==\frac{-y.z.x}{x.y.z}=-1\)