a) $$ \frac{4b}{(a-b)(a+b)} + \frac{a-b}{a(a+b)} = \frac{4ab + (a-b)^2}{a(a-b)(a+b)} = \frac{4ab + a^2 - 2ab + b^2}{a(a-b)(a+b)} = \frac{a^2 + 2ab + b^2}{a(a-b)(a+b)} = \frac{(a+b)^2}{a(a-b)(a+b)} = \frac{a+b}{a(a-b)} $$
Ответ: $$ \frac{a+b}{a(a-b)} $$