Giải thích các bước giải:
$a)$ $\dfrac{a}{b}$ $-$ $\dfrac{c}{d}$ $=$ $\dfrac{a}{b}$ $.$ $\dfrac{c}{d}$
$⇔$ $\dfrac{ad - bc}{bd}$ $=$ $\dfrac{ac}{bd}$
$⇔$ $ad - bc = ac$
$⇔$ $ad = ac + bc ⇔ ad = c(a + b)$
$⇔$ $\dfrac{c}{d}$ $=$ $\dfrac{a}{a + b}$
$\text{#Annihilators}$