a) Ta có: $\frac{a}{b}=\frac{c}{d}$ nên $\frac{a}{b} +1=\frac{c}{d}+1$, suy ra $\frac{a}{b}+\frac{b}{b}=\frac{c}{d}+\frac{d}{d}$ hay $\frac{a+b}{b}=\frac{c+d}{d}$
b)Ta có: $\frac{a}{b}=\frac{c}{d}$ nên $\frac{a}{b} -1=\frac{c}{d}-1$, suy ra $\frac{a}{b}-\frac{b}{b}=\frac{c}{d}-\frac{d}{d}$ hay $\frac{a-b}{b}=\frac{c-d}{d}$