\[\left( \frac{a^{\backslash a}}{a - b} - \frac{a + b^{\backslash a - b}}{a} \right) \cdot \frac{a - b}{2b} =\]
\[= \frac{a^{2} - a^{2} + b^{2}}{a(a - b)} \cdot \frac{a - b}{2b} = \frac{b²(a - b)}{2ab(a - b)} = \frac{b}{2a}\]