\[\boxed{\text{143\ (143).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[\textbf{а)}\ \frac{1}{a} + \frac{1}{b} = \frac{1}{c}\]
\[\frac{1}{c} = \frac{1^{\backslash b}}{a} + \frac{1^{\backslash a}}{b}\]
\[1\ :c = \frac{b + a}{\text{ab}}\]
\[c = 1\ :\frac{b + a}{\text{ab}} = 1 \cdot \frac{\text{ab}}{b + a}\]
\[c = \frac{\text{ab}}{a + b}.\]
\[\textbf{б)}\ \frac{1}{a} + \frac{1}{b} = \frac{1}{c}\]
\[\frac{1}{b} = \frac{1^{\backslash a}}{c} - \frac{1^{\backslash c}}{a}\]
\[1\ :b = \frac{a - c}{\text{ac}}\]
\[b = 1\ :\frac{a - c}{\text{ac}} = 1 \cdot \frac{\text{ac}}{a - c}\]
\[b = \frac{\text{ac}}{a - c}.\]