If V_A and V_B denote potentials of A and B, then the equivalent resistance between A and B in the adjoining circuit is ( the diode is ideal)

 

$\begin{array}{rcl} & & \mathrm{If} {\mathrm{V}}_{\mathrm{A}} \mathrm{is} \mathrm{greater} \mathrm{than} {\mathrm{V}}_{\mathrm{B}, }\mathrm{the} \mathrm{diode} \mathrm{is} \mathrm{in} \mathrm{forward} \mathrm{bias} \mathrm{and} \mathrm{effective} \mathrm{resistance} \\ & & \mathrm{of} \mathrm{the} \mathrm{circuit} \mathrm{is} 50\left|\right|50 \mathrm{is} 25 \mathrm{ohm} \mathrm{and}\\ & & \mathrm{if} {\mathrm{V}}_{\mathrm{A}} \mathrm{is} \mathrm{less} \mathrm{than} {\mathrm{V}}_{\mathrm{B},} \mathrm{diode} \mathrm{is} \mathrm{in} \mathrm{reverse} \mathrm{bias} \mathrm{and} \mathrm{effective} \mathrm{resistance} \\ & & \mathrm{of} \mathrm{the} \mathrm{circuit} \mathrm{is} 50 \mathrm{ohm}\end{array}$