For the stress system shown,

$4\mathrm{.}1$ Calculate from first principles, the value of the normal and shear stress on the plane $AB$

$(6)$

$4\mathrm{.}2$ Calculate the principal stresses and the maximum shear stress for the system $($using standard formulae$)$

$(5)$

$4\mathrm{.}3$ Check your results for $4\mathrm{.}1,$ and $4\mathrm{.}2$ using Mohr's circle

$(17)$