Finding small signal resistance for MOSFET
You just need to apply the usual method to determine the resistance seen from two terminals, here being \$v_o\$ (gate-drain terminal) and ground. Turn off all the sources. The circuit is







simulate this circuit &ndash; Schematic created using CircuitLab

Calculate \$i_x\$ and calculate vx/ix i.e. \$r_{d1}\$, which should be trivial.