б) $$\frac{1}{5a^2+10ab+20b^2} \cdot (a^3-8b^3) = \frac{(a-2b)(a^2+2ab+4b^2)}{5(a^2+2ab+4b^2)} = \frac{a-2b}{5}$$
Ответ: (a-2b)/5