I think that the sin(theta) and cos(theta) should not have been omitted in the last two equations for pressure in the x direction and pressure in the z direction, although the cos term will divide out since the sum of forces in the x direction equals zero and the sin term will divide out in a similar fashion when delta z becomes infinitesimally small. Am I correct by saying this? Is this just a simplification that was understood?
+Robert Kerestes cos(theta) and sin(theta) originally appear as they are used to resolve P_n into an x and a z component. The delta s * b term appears as it is associated with the area of the surface over which the pressure acts. The cos(theta) and sin(theta) terms later "disappear" from the equation as they become coupled with delta s term and translate into delta z and delta x. This is not a small angle approximation, rather just application of trigonometry.
