$3\mathrm{.}5.$ For an isotropic elastic medium subjected to a hydrostatic

state of stress, ${}_{}={}_{yy}={}_{zz}=-p$ and ${}_{xy}={}_{xz}={}_{yz}=0,$

where $p$ denotes pressure $F{L}^{-2}.$ Show that for this state of

stress $p=-Ke,$ where $K=\frac{E}{3(1-2v)}$ is the bulk modulus and

$e=lo{n}_{}+lo{n}_{yy}+lo{n}_{zz}$ is the classical small$-$displacement cubical

strain $($also called the volumetric strain$).$