Series and Parallel

Problem 1:

Find $R_{ab}$?

Solution: $R_{ab}=R_1+R_2$

Problem 2:

Find $R_{ab}$?

Solution: $\frac{1}{R_{ab}}=\frac{1}{R_1}+\frac{1}{R_2}$

               $R_{ab}=\frac{R_1R_2}{R_1+R_2}$