Find $\displaystyle f'(a)$

$\displaystyle

f(x)=\frac {1} {\sqrt{x+2}}

$

So I use the formula $\displaystyle f'(a)=\lim_{h\to0}\frac {f(a+h)-f(a)} {h}$ and get $\displaystyle \frac {\frac {1} {\sqrt{a+h+2}}-\frac {1} {\sqrt{a+2}}} {h}$. My Maple 13 calculator says this cannot be simplified unless you throw in the limit ashapproaches 0, which gets

$\displaystyle

-\frac {1} {2(a+2)^{3/2}}

$

How does it reach this answer? Thanks to anyone who helps!