bài 3 :
a:
\(\dfrac{x}{7}=\dfrac{-4}{14}\Rightarrow x\cdot14=7\cdot\left(-4\right)=-28\\ \Rightarrow x=\left(-28\right):14\\ \Rightarrow x=-2\)
b: \(\dfrac{x}{2}=\dfrac{-12}{6}\Rightarrow x\cdot6=2\cdot\left(-12\right)=-24\\ \Rightarrow x=-24:6\\ \Rightarrow x=-4\)
c: \(\dfrac{x}{9}=\dfrac{-45}{81}\Rightarrow x\cdot81=9\cdot\left(-45\right)=-405\\ \Rightarrow x=-405:81\\ \Rightarrow x=5\)
d: \(\dfrac{7}{x}=\dfrac{-42}{48}\Rightarrow7\cdot48=x\cdot\left(-42\right)=336\\ \Rightarrow x=8\)
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