4. $$rac{4x}{4x-4} + \frac{5x}{7x-7} =$$

$$\frac{4x}{4x-4} + \frac{5x}{7x-7} = \frac{4x}{4(x-1)} + \frac{5x}{7(x-1)} = \frac{x}{x-1} + \frac{5x}{7(x-1)} = \frac{7x+5x}{7(x-1)} = \frac{12x}{7(x-1)}$$ $$\textbf{Ответ: } \frac{12x}{7(x-1)}$$