Giải thích các bước giải:
a.Ta có:
$A=\dfrac{5x+3y}{6x-7y}$
$\to A=\dfrac{(5x+3y):y}{(6x-7y):y}$
$\to A=\dfrac{5\cdot \dfrac{x}{y}+3}{6\cdot \dfrac{x}{y}-7}$
$\to A=\dfrac{5\cdot\dfrac23+3}{6\cdot \dfrac23-7}$
$\to A=-\dfrac{19}{3}$
b.Ta có:
$\dfrac{x}{y}=\dfrac35\to x=\dfrac35y$
$\to B=\dfrac{\dfrac35y+y}{\dfrac35y-y}$
$\to B=\dfrac{\dfrac85y}{\dfrac{-2}5y}$
$\to B=-4$