>>11405734b) Let S be the infinitesimal box with side lengths 2dx,2dy,2dz centered at p.Use first order approximations for B at the center of the faces:
B(p +/- dx) = B(p) +/- dx*(d/dx)B(p)
B(p +/- dy) = B(p) +/- dy*(d/dy)B(p)
B(p +/- dz) = B(p) +/- dz*(d/dz)B(p)
and do the cross product with the normals of the faces (remember the magnitude of the normal is the area of the face).
Verify the answer is the same as (del x B)(p)*8*dx*dy*dz
c) do something similar with an infinitesimal square
