The beam shown in cross-section is made of a steel assumed to be elastoplastic (E-200 GPa and oy-300 MPa), for bending around the z-axis, (a) at the moment when yielding begins, (b) at the moment when the thickness of the plastic zones above and below the bar is 25 mm. Determine the bending moment. 2 12.5 mm- 50 mm 25 mm 50 mm 25 mm 12.5 mm