a, Ta có: \(7y=5z\Leftrightarrow\dfrac{y}{5}=\dfrac{z}{7}\)
Ta lại có: \(\dfrac{x}{3}=\dfrac{y}{4}\Leftrightarrow\dfrac{x}{15}=\dfrac{y}{20}\left(1\right)\)
\(\dfrac{y}{5}=\dfrac{z}{7}\Leftrightarrow\dfrac{y}{20}=\dfrac{z}{28}\left(2\right)\)
Từ \(\left(1\right);\left(2\right)\Rightarrow\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}\Leftrightarrow\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}\) và \(2x+3y-z=186\)
Áp dụng tính chất dãy tỉ số bằng nhau ta có:
\(\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}=\dfrac{2x+3y-z}{30+60-28}=\dfrac{186}{62}=3\)
+) \(\dfrac{2x}{30}=3\Rightarrow2x=3.30=90\Rightarrow x=90:2=45\)
+) \(\dfrac{3y}{60}=3\Rightarrow3y=3.60=180\Rightarrow y=180:3=60\)
+) \(\dfrac{z}{28}=3\Rightarrow z=3.28=84\)
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