121. (\(\frac{1}{a} + \frac{1}{b}\)) \cdot \(\frac{2ab}{a^2 - b^2}\).

121. Упростим выражение:

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

Ответ: \(\frac{2}{a - b}\)