هر یک از موارد زیر را ثابت کنید. الف) $\frac{a}{b} = \frac{c}{d} \implies ad = bc$ ب) $ad = bc \implies \frac{a}{b} = \frac{c}{d}$ پ) $\frac{a}{b} = \frac{c}{d} \implies \frac{a}{c} = \frac{b}{d}$ ت) $\frac{a}{b} = \frac{c}{d} \implies \frac{a+b}{c+d} = \frac{a}{c} = \frac{b}{d}$ ث) $\frac{a}{b} = \frac{c}{d} \implies \frac{a-b}{c-d} = \frac{a}{c} = \frac{b}{d}$