AD t/c dãy tí số = nhau
$\dfrac{x}{3}$=$\dfrac{y}{4}$=$\dfrac{z}{5}$ \Rightarrow $\dfrac{x}{3}$ = $\dfrac{2y}{8}$ = $\dfrac{3z}{15}$ = $\dfrac{x+2y-3z}{3+8-15}$ = $\dfrac{x-2y+3z}{3-8+15}$
\Rightarrow $\dfrac{x+2y-3z}{-4}=\dfrac{x-2y+3z}{10}$
\Rightarrow$\dfrac{x+2y-3z}{x-2y+3z}=\dfrac{-2}{5}$