Quote by Doc Al
I'd say that was wrong. What book are you using?

This one, third page.
Quote by Doc Al
For example:
[tex]\sqrt{z^2 + R^2} = z\sqrt{1 + (R/z)^2}[/tex]
Since R/z << 1, can you see how to use a binomial approximation now?

You mean the approximation is ~= 1+z^2/2*R^2?