\[{\text{Если }}\frac{a}{b} = \frac{c}{d}{\text{, то }}\frac{{a + b}}{b} = \frac{{c + d}}{d};{\text{ }}\frac{a}{{a + b}} = \frac{c}{{c + d}};{\text{ }}\frac{{a + b}}{{a - b}} = \frac{{c + d}}{{c - d}}.\]