A beam of unsymmetric cross section is subjected to a couple M0 acting in the horizontal plane xz. Show that the stress at point A, of coordinates y and z, is
 
where Iy Iz., and Iyz. denote the moments and product of inertia of the cross section with respect to the coordinate axes, and Mz. the moment of the couple.
 

yl, . M. 1,1. 1