Vì $x-y-z=0$
$\Rightarrow\left\{{}\begin{matrix}x-y=z\\x-z=y\\y+z=x\end{matrix}\right.$
Thay vào biểu thức `H` ta có :
$H=\left(1-\dfrac{z}{x}\right)\left(1-\dfrac{x}{y}\right)\left(1+\dfrac{y}{z}\right)$
$⇒H=\dfrac{x-z}{x}.\dfrac{y-x}{y}.\dfrac{y+z}{z}$
$⇒H=\dfrac{y}{x}.\dfrac{-z}{y}.\dfrac{x}{z}$
$⇒H=-1$