Problem $2(10$ points$).$ The beam section shown below is an ellipse with $a=80mm$ and $b=40$

mm $.$ The bending moment $M_z$ resisted by this section is $50N*m.$ Determine the maximum bending stress

in the section. The expression for the moment of inertia about $z,I_z=\frac{}{4}a{b}^3.$