1. The problem statement, all variables and given/known data Hi,I am trying to understand the proof attached. My problem is shown by a red arrow.Can someone explain those 2 steps? And please answer it as simply as you can...Since I haven't done multivariable calculus..It is a physics course.Thanks 2. Relevant equations 3. The attempt at a solution Let say the integral of A(r') 。d(r') is L(r),so if you take the definite integral it becomes L(r+dr) - L(r),but in the proof they said dr is infinitesimal so dr = 0,so L(r+dr) - L(r) = 0 =/= A(r) 。d(r) ?