$2\mathrm{.}3-2.$ Determine the principal stresses and the corresponding directions of cosine for the

following state of stress $($in MPa$).$ Also verify that the stress invariants I$2$ and I$3$ will remain the

same.

$($b$){}_x=-80,{}_y=40,{}_z=-40,{}_{xy}=-40,{}_{yz}=120,{}_{zx}=80$

$($d$){}_x=180,{}_y=120,{}_z=-80,{}_{xy}=-140,{}_{yz}=80,{}_{zx}=110$