The V-I characteristic of a silicon diode is given in fig. calculate the diode resistance in forward bias at  $V=+2V\text{ and V=+1V}$. 

The forward bias diode resistance is given by $r_{fb}=\frac{\Delta V}{\Delta I}$                  
$Where\Delta V\text{ and }\Delta \text{I }$ Are the small changes in the voltage and current near the desired values.
$$\begin{gathered} r_{fb}(at+2V)=\frac{(2.2-1.8)V}{(80-60)mA}=\frac{0.4V}{20\times {10}^{-3}A}=20\Omega \\ {r}_{fb}(at+1V)=\frac{(1.2-0.8)V}{(20-10)mA}=\frac{0.4V}{10\times {10}^{-3}A}=40\Omega \end{gathered}$$