dynamic resistance of PN junction diode
My take on this question: 

Assume that: $$R &gt;&gt; r_{ac}$$ This is a reasonable assumption if E >> the threshold voltage of the diode. 

V2 = only the AC component of the voltage measurement 

$$r_{ac} = \frac{v_{ac}}{i_{ac}} = \frac{V_2}{\frac{V_1}{R_x + r_{ac}}}$$

$$r_{ac} = V_2.\frac{(R_x + r_{ac})}{V_1}$$

$$r_{ac}.V_1 = V_2.(r_{ac} + R_x)$$

$$r_{ac}.V_1 = r_{ac}.V_2 + R_x.V_2$$

$$r_{ac} = \frac{V_2.R_x}{(V_1 - V_2)}$$