4. Сократите дробь: 2) (c+9√c)/(c-81);

2) \( \frac{c + 9\sqrt{c}}{c - 81} = \frac{\sqrt{c}^2 + 9\sqrt{c}}{\sqrt{c}^2 - 9^2} = \frac{\sqrt{c}(\sqrt{c} + 9)}{(\sqrt{c} - 9)(\sqrt{c} + 9)} = \frac{\sqrt{c}}{\sqrt{c} - 9} \). Ответ: \( \frac{\sqrt{c}}{\sqrt{c} - 9} \).