We know that,Resistance of diode is given vy \\\\(\\cfrac{\ riangle V}{\ riangle I}\\\\)So, at \\\\(I_V=15 mA\\\\)From \\\\(V-I\\\\)characteristic equation:\\\\(R=\\cfrac{\\left(0.8-0.7\\right)V}{\\left(20-10\\right)mA}=10\\Omega\\\\)And,at \\\\(V_n=-10V\\\\)\\\\(\ riangle I=0\\\\)So, \\\\(R\\\\) is infinite.