This installment of “It’s Obvious. Not!” looks at: Book: “div grad curl and all that” Edition: second Author: H. M. Schey Publisher W. W. Norton. Page: 95 As is often the case in this excellent book, the author illustrates theory with a concrete example. The example demonstrates that Stokes Theorem works for a vector field described by: where Stokes Theorem is evaluated using the path of the circumference of a unit circle in the x-y plane and using the surface enclosed by the unit circle on the x-y plane. I ran into a few minor points of confusion working through the example, and I’ve added my intermediate steps below. When evaluating the line integral, the book immediately moves from: immediately to: My confusion: What happened to the dx and the dz terms? Remembering that the path lies entirely in the x-y plane, z is always equal to 0. So, the dx term above drops out. Also, because z is a constant value in the x-y plane, x dz always evalua...