Here is a single point stress tensor:

$1-$ derive the stress vector on the surface parallel to x$+$y$+$z$=1$

$2-$ Derive the normal and tangential components on a surface with unit vector $(1/2,1/2,1/$sqrt$(2))$

$=\left[\begin{array}{ccc}15& -5& 0\\ -5& 20& 10\\ 0& 10& 18\end{array}\right]$

for second part?

$(\frac{1}{2},\frac{1}{2},\frac{1}{\sqrt[\phantom{2}]{2}})$