I'm not getting the algebric trick to do this limit, when $\displaystyle a>>x$

$\displaystyle \frac{1}{a}\ln(\frac{\sqrt{a^2+x^2} + a}{\sqrt{a^2+x^2} - a})$

In a similar question i did the taylor expansion with 2 terms: $\displaystyle (1+b)^n=1+nb $ for $\displaystyle b<<1$, and it worked fine (i that case $\displaystyle b=\frac{x^2}{a^2}$). Probably i'll have to use the same in here.

Thank's.