I am not a student, but I am trying to figure this out.

The text book states:

"We expand our previous forma(using a binomial expansion) and drop 'higher order' terms"

The formula the text is referring to is

$\displaystyle \frac{F}{1-dF}$

And after binomial expansion reduces to:

$\displaystyle F^2 \time d$

I appreciate any help.