Series and Parallel circuits practice exercise

Problem 1:

Find $R_{ab}$=?

        $R_{cd}$=?

        $R_{ac}$=?

Solution: $R_{ab}=\left(R_1+R_3\right)=R_{ad}$

               $R_{cd}=\left(R_1+R_3\right)=R_{cb}$

               $R_{ac}=\left(R_1+R_2\right)$