Electrical Resistance If two electrical resistors with resistances $R_{1}$ and $R_{2}$ are connected in parallel (see the figure), then the total resistance $R$ is given by
$$R=\frac{1}{\frac{1}{R_{1}}+\frac{1}{R_{2}}}$$
(a) Simplify the expression for $R$
(b) If $R_{1}=10$ ohms and $R_{2}=20$ ohms, what is the total resistance $R ?$