$4.$ Optional Exercise. $($Value: $1$ additional point toward the final grade$).$ Based on the following exercise, for a given stress state, determine a$)$ the normal stresses and b$)$ the shear stresses acting on the oblique face of the shaded triangular element shown in Figure $4.$ Use an analysis method based on the equilibrium equations of the triangular element.

Figure $4.$ Stress State with Triangular Element.