А. a=bc/(b+c)
Б. a=bc/(c-b)
В. a=(b+c)/bc
Г. a=bc/(b-c)
\[\frac{1}{c} = \frac{1}{a} - \frac{1}{b}\]
\[\frac{1}{a} = \frac{1}{c} + \frac{1}{b}\]
\[\frac{1}{a} = \frac{b + c}{\text{bc}}\]
\[a = \frac{\text{bc}}{b + c}\]
\[Ответ:\ \ а).\]