20. $$\frac{a-2}{a^2-9} + \frac{3}{a^2+3a} =$$

$$\frac{a-2}{a^2-9} + \frac{3}{a^2+3a} = \frac{a-2}{(a-3)(a+3)} + \frac{3}{a(a+3)} = \frac{a(a-2)}{a(a-3)(a+3)} + \frac{3(a-3)}{a(a-3)(a+3)} = \frac{a^2-2a+3a-9}{a(a-3)(a+3)} = \frac{a^2+a-9}{a(a-3)(a+3)}$$ Ответ: $$\frac{a^2+a-9}{a(a-3)(a+3)}$$