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 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: $$\sqrt{z^2 + R^2} = z\sqrt{1 + (R/z)^2}$$ 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?