\[\boxed{\text{166\ (166).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \frac{x - a}{x - b}\text{\ \ }при\ x = \frac{\text{ab}}{a + b}\]
\[= \frac{a(b - a - b)}{a + b} \cdot \frac{a + b}{b(a - a - b)} =\]
\[= \frac{- a^{2}}{- b^{2}} = \frac{a^{2}}{b^{2}}\text{.\ }\]
\[\textbf{б)}\ \frac{\frac{a}{b} - x}{\frac{b}{a} + x}\ \ \ при\ x = \frac{a - b}{a + b}\]