a) \(\dfrac{x}{21}=\dfrac{y}{-5}=\dfrac{x+y}{21+\left(-5\right)}=\dfrac{32}{17}\)
\(\Rightarrow\dfrac{x}{21}=\dfrac{32}{17}\Rightarrow x=\dfrac{32.21}{17}=\dfrac{672}{17}\)
\(\Rightarrow\dfrac{y}{-5}=\dfrac{32}{17}\Rightarrow y=\dfrac{32.\left(-5\right)}{17}=\dfrac{-160}{17}\)
b) \(\dfrac{x}{y}=\dfrac{-7}{4}\)
\(\Leftrightarrow4x=-7y\) (1)
\(\Leftrightarrow4x+7y=0\)
\(\Leftrightarrow4x-5y+12y=0\)
\(\Leftrightarrow72+12y=0\)
\(\Leftrightarrow12y=-72\)
\(\Leftrightarrow y=-6\) (2)
Thế (2) vào (1) có: \(4x=\left(-7\right).\left(-6\right)\Leftrightarrow4x=42\Leftrightarrow x=10,5\)
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