toán khó nha

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0973573959thuy

a/b=3/4 \Rightarrow $a = \frac{3b}{4}$
$\frac{2a - 5b}{a - 3b} = \frac{2.\frac{3b}{4} - 5b}{\frac{3b}{4} - 3b} = \frac{\frac{3b}{2} - 5b}{\frac{-9b}{4}} = \frac{-7b}{2} : \frac{-9b}{4} = 1\frac{5}{9}$
 
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