1. The problem statement, all variables and given/known data I'm currently following the textbook Advanced Engineering Mathematics by Erwin Kreyszig. I'm learning the derivation of the Wave equation using the method shown in the book, but when I reached the final part of the derivation, the working just confuses me. (1/Δx)[ (u/dx)| x+Δx - (du/dx)| x ] = (p/T) ( second derivative of u wrt t ) at this part, it mentions that we are taking the lim ∆x→0, which will end up in the wave equation. 2. Relevant equations (1/Δx)[ (u/dx)| x+Δx - (du/dx)| x ] 3. The attempt at a solution I've tried working out the limit, but I guess there's something I don't know since as Δx approaches zero, the derivative terms will become the same and cancel out each other?