$*17.$ Find the Th$$venin equivalent circuit for the portion of the networks of Fig. $19\mathrm{.}121$ external to the elements between points a and $b.$

FIG. $19\mathrm{.}121$

Problem $35.$

Find the Norton equivalent circuit for the network external to the elements between a and b for the network of Fig$\mathrm{.}19\mathrm{.}121.$