Hi guys, This is my first time posting on PF! I have a question on §3 of Einstein's paper "On the electrodynamics of moving bodies." My problem is with the following mathematical statements: Hence, if x' is chosen to be infinitesimally small, or I have just finished highschool, and whilst I have finished two other books which concerned Special Relativity (and understood them well) I think I do not know enough maths to understand what i pasted above. Could someone explain how he gets from step 1 to step 2 please? Or, perhaps could someone tell me what topics in calculus I must learn in order to understand it myself? Thanks!! EDIT: I found many other threads asking the exact same thing as me... Oops! I managed to derive it on my own so no answer is necessary :).