a)
Ta có : $\dfrac{a}{b}=\dfrac{c}{d}$
$⇒\dfrac{a}{b}+1=\dfrac{c}{d}+1$
$⇒\dfrac{a}{b}+\dfrac{b}{b}=\dfrac{c}{d}+\dfrac{d}{d}$
$⇒\dfrac{a+b}{b}=\dfrac{c+d}{d}$
b)
Ta có : $\dfrac{a}{b}=\dfrac{c}{d}$
$⇒a.d=b.c$
$⇒\dfrac{a}{c}=\dfrac{b}{d}$
$⇒\dfrac{a}{c}-1=\dfrac{b}{d}-1$
$⇒\dfrac{a}{c}-\dfrac{c}{c}=\dfrac{b}{d}-\dfrac{d}{d}$
$⇒\dfrac{a-c}{c}=\dfrac{b-d}{d}$
