\[\frac{1}{c} = \frac{1}{c_{1}} + \frac{1}{c_{2}}\]
\[\frac{1}{c} = \frac{c_{2} + c_{1}}{c_{1}c_{2}}\]
\[c = \frac{1}{\frac{c_{2} + c_{1}}{c_{1}c_{2}}}\]
\[c = \frac{c_{1}c_{2}}{c_{2} + c_{1}}.\]