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## Homework Statement

I am trying to expand [itex]\frac{1}{(1+\frac{a^2}{z^2})^{1/2}}[/itex] for z>>a.

## Homework Equations

## The Attempt at a Solution

First, I rearranged the equation to [itex](1+\frac{a^2}{z^2})^{-1/2}[/itex]. After this, since z>>a, can I assume z takes a value of infinity and say the first term is 1+0=1? And I am not sure what to do for the second term. I take the first derivative which is [itex]-\frac{1}{2}(1+\frac{a^2}{z^2})^{-3/2}(\frac{-2a^2}{z^3})[/itex] and not sure what to do with it.

Any help will be appreciated.