\(a,\) Áp dụng t/c dtsbn
\(\dfrac{x}{10}=\dfrac{y}{6}=\dfrac{z}{21}=\dfrac{5x}{50}=\dfrac{2z}{42}=\dfrac{5x+y-2z}{50+6-42}=\dfrac{28}{14}=2\\ \Rightarrow\left\{{}\begin{matrix}x=20\\y=12\\z=42\end{matrix}\right.\)
\(b,\) Áp dụng t/c dtsbn
\(\dfrac{x}{8}=\dfrac{y}{-7}=\dfrac{z}{21}=\dfrac{3x}{24}=\dfrac{10y}{-70}=\dfrac{2z}{42}=\dfrac{-3x+10y-2z}{-24-70-42}=\dfrac{236}{-136}=-\dfrac{59}{34}\\ \Rightarrow\left\{{}\begin{matrix}x=-\dfrac{236}{17}\\y=\dfrac{413}{34}\\z=-\dfrac{1239}{34}\end{matrix}\right.\)
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