$1\mathrm{.}81.$ The stresses at a point in a loaded machine bracket with respect to the $x,y,z$ axes are given

as

$\left[\begin{array}{ccc}36& 0& 0\\ 0& 48& 0\\ 0& 0& -72\end{array}\right]$MPa

Determine $($a$)$ the octahedral stresses and $($b$)$ the maximum shearing stresses.