From David J. Griffiths "Introduction to Electrodynamics 2. ed":

"The third term" is further up the page.Quote:

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