Cách làm cho bạn:
a) Ta có:
$\frac{a}{b}=\frac{c}{d}$
$=>\frac{a}{b}+1 = \frac{c}{d}+1$
=> $\frac{a}{b}+\frac{b}{b}=\frac{c}{d}+\frac{d}{d}$
hay $\frac{a+b}{b}=\frac{c+d}{d}$
b)Ta có: $\frac{a}{b}=\frac{c}{d}$
=> $\frac{a}{b}−1 = \frac{c}{d}−1$,
=> $\frac{a}{b}−\frac{b}{b}=\frac{c}{d}−\frac{d}{d}$
hay $\frac{a−b}{b}=\frac{c−d}{d}$
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