122. \(\left(\frac{1}{a}-\frac{1}{b}\right):\frac{b^2-a^2}{ab^2}\)

\(\frac{1}{a}-\frac{1}{b}=\frac{b-a}{ab}\), а \(b^2-a^2=(b-a)(b+a)\).

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

Ответ: \(\frac{b}{a+b}\).