From David J. Griffiths "Introduction to Electrodynamics 2. ed":
"The third term" is further up the page.We're interested in the regime (...), so the third term is necligible, and the binomial expansion yields
What is really going on here? I've been trying to understand it, but I can't figure it out. How does he get ?
Edit/context:
This is a step in deriving the formula
which in turn leads to