a/

Theo đề,ta có:

+/ \(\dfrac{x}{2}=\dfrac{y}{3}\Rightarrow\dfrac{x}{8}=\dfrac{y}{12}\left(1\right)\)

+/\(\dfrac{y}{4}=\dfrac{z}{5}\Rightarrow\dfrac{y}{12}=\dfrac{z}{15}\)\(\left(2\right)\)

Từ (1) và (2), ta có:

\(\dfrac{x}{8}=\dfrac{y}{12}=\dfrac{z}{15}\)

Áp dụng tính chất của dãy tỉ số bằng nhau, ta có:

\(\dfrac{x}{8}=\dfrac{y}{12}=\dfrac{z}{15}=\dfrac{x-y-z}{8-12-15}=\dfrac{28}{-19}\)

Do đó:

+/ \(\dfrac{x}{8}=\dfrac{28}{-19}\Rightarrow x=-\dfrac{224}{19}\)

+/\(\dfrac{y}{12}=\dfrac{28}{-19}\Rightarrow y=-\dfrac{336}{19}\)

+/\(\dfrac{z}{15}=\dfrac{28}{-19}\Rightarrow z=-\dfrac{420}{19}\)

Vậy: + \(x=-\dfrac{224}{19}\)

+ \(y=-\dfrac{336}{19}\)

+ \(z=-\dfrac{420}{19}\)