a) Đặt \(\dfrac{x}{12}=\dfrac{y}{9}=\dfrac{z}{5}=k\)
\(\Rightarrow x=12k;y=9k;z=5k\)
Ta có 
:\(xyz=12k.9k.5k=20\)
\(540.k^3=20\)
\(\Rightarrow k^3=\dfrac{1}{27}\)
\(\Rightarrow k=\dfrac{1}{3}\)
\(\Rightarrow x=12.\dfrac{1}{3}=4\)
\(\Rightarrow y=9.\dfrac{1}{3}=3\)
\(\Rightarrow z=5.\dfrac{1}{3}=\dfrac{5}{3}\)
Vậy \(x=4 ; y=3 ; z=5/3\)
Đặt:
\(\dfrac{x}{12}=\dfrac{y}{9}=\dfrac{z}{5}=k\)
\(\Leftrightarrow x=12k;y=9k;z=5k\)
\(xyz=20\Leftrightarrow12k.9k.5k=20\)
\(k\left(12.5.9\right)=20\)
\(540k=20\)
\(k=\dfrac{1}{27}\)
\(x=\dfrac{1}{27}.12=\dfrac{4}{9}\)
\(y=\dfrac{1}{27}.9=\dfrac{1}{3}\)
\(z=\dfrac{1}{27}.5=\dfrac{5}{27}\)
