The area of the inclined surface is:

${\mathrm{A}}_{\mathrm{n}}=\mathrm{A} / \cos \mathrm{\theta }$

The shear stress on the inclined surface (i.e., the $\mathrm{n}$ face) in the $\mathrm{t}$ direction is:

$$\begin{gathered} {\mathrm{\tau }}_{\mathrm{nt}}=\mathrm{V} / {\mathrm{A}}_{\mathrm{n}} \\ =(-\mathrm{P} \sin \mathrm{\theta }) / (\mathrm{A}/\cos \mathrm{\theta }) \\ =-\mathrm{P}/\mathrm{A} \sin \mathrm{\theta } \cos \mathrm{\theta } \\ =-{\mathrm{\sigma }}_{\mathrm{x}} \sin \theta \cos \mathrm{\theta } \end{gathered}$$

where ${\mathrm{\sigma }}_{\mathrm{x}}=\mathrm{P}/\mathrm{A}=$ the normal stress on a vertical plane (i.e., the $\mathrm{x}$ face).