$(3.)$ A beam is made by gluing four rectangular planks together.

a$)$ Determine the magnitude and direction of the maximum shearing stress.

$s=7cm$

$v=4cm$

$l=2\mathrm{.}3m$

$Q=195kN$

b$)$ What does the reciprocity of shear stresses $($complementary property of shear$)$ state? Which equilibrium equation of the differential volume element proves the ${}_{xz}={}_{zx}$ equality?