89. 1) $$(1 - \frac{a-b}{a+b}) \cdot (2 + \frac{2b}{a-b})$$

Решение:

1. $$1 - \frac{a-b}{a+b} = \frac{a+b-(a-b)}{a+b} = \frac{a+b-a+b}{a+b} = \frac{2b}{a+b}$$
2. $$2 + \frac{2b}{a-b} = \frac{2(a-b)+2b}{a-b} = \frac{2a-2b+2b}{a-b} = \frac{2a}{a-b}$$
3. $$\frac{2b}{a+b} \cdot \frac{2a}{a-b} = \frac{4ab}{(a+b)(a-b)} = \frac{4ab}{a^2 - b^2}$$

Ответ: $$\frac{4ab}{a^2 - b^2}$$