Ta có : \(\dfrac{a}{b}=\dfrac{3}{4}\Leftrightarrow\dfrac{a}{3}=\dfrac{b}{4}\\ Đặt\dfrac{a}{3}=\dfrac{b}{4}=k\Rightarrow\left\{{}\begin{matrix}a=3k\\b=4k\end{matrix}\right.\\ ThayvàoA,tacó:\)
\(A=\dfrac{2a-5b}{a-3b}-\dfrac{4a+b}{8a-2b}\\ \Leftrightarrow=\dfrac{2\cdot3k-5\cdot4k}{3k-3\cdot4k}-\dfrac{4\cdot3k+4k}{8\cdot3k-2\cdot4k}\\ =\dfrac{6k-20k}{3k-12k}-\dfrac{12k+4k}{24k-8k}\\ =\dfrac{14k}{9k}-\dfrac{16k}{16k}\\ =\dfrac{14}{9}-1\\ =\dfrac{5}{9}\)
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