Решение

1. $$\frac{a}{b} - 1 = \frac{a - b}{b}$$
2. $$\frac{8}{x^2} - \frac{3}{x} + 2 = \frac{8 - 3x + 2x^2}{x^2} = \frac{2x^2 - 3x + 8}{x^2}$$
3. $$5 - \frac{4m + 5n}{n} = \frac{5n - (4m + 5n)}{n} = \frac{5n - 4m - 5n}{n} = \frac{-4m}{n}$$

Ответ:

1. $$\frac{a - b}{b}$$
2. $$\frac{2x^2 - 3x + 8}{x^2}$$
3. $$\frac{-4m}{n}$$