Q$\mathrm{.}4($b$)$ A curved bar with a rectangular cross section is subjected to a bending moment of $M=4500$ Nm $,$ acting in the direction shown. The bar has a width of $35$ mm and a height of $70$ mm $.$ The inside radius of the curved bar is $50$ mm $.$ Determine the bending stresses in the curved bar at points A and $B.$