Find the first three terms, in ascending powers of $x$, of the binomial expansion of

$\frac{1}{\sqrt{9+x}}$

giving each coefficient in its simplest form.

A student wants to use this expansion to obtain an approximation for $\sqrt{3}$, and notes that each of the three values $x=3$, $x=-6$ and $x=18$ leads to an expression involving $\sqrt{3}$.

Without evaluating your expansion, state, giving a reason, which one of the three values of $x$ should not be used.

Without evaluating your expansion, state, giving a reason, which of the remaining two values of $x$ would lead to the more accurate approximation.