6) $$\frac{a+5}{b-4} + \frac{a}{4-b} =$$

$$\frac{a+5}{b-4} + \frac{a}{4-b} = \frac{a+5}{b-4} - \frac{a}{b-4} = \frac{a + 5 - a}{b-4} = \frac{5}{b-4}$$

Ответ: $$\frac{5}{b-4}$$