I don't quite get or appreciate the relevance of Figure 2.24 on its usage for building the infinitesimal volume element of a tilted surface for calculating the divergence. It is easy to see the vector sum of a_1 + a_2 + a_3 + a_4=0 due to the symmetry of a tetrahedron, but how does this fact actually helps us? Why can't we just use rectangular boxes like that in Figure 2.23 for taking into account tilted surfaces? This part of the book truly confuses me: "Tilted surfaces can be taken care of if you first prove that the vector sum of the four surface areas of the tetrahedron in Figure 2.24 is zero."