Chemistry - Equilibria questions involving ICE tables

[EdTheHead]

In my book for all these equilibria questions involving ice tables when I end up with Ka = \frac{(x)(x)}{initial amount - x} they always say that if the Ka is tiny compared to the initial amounts then we can assume x will be too and we can omit as many x's as we like from this equation. This obviously makes solving for x a whole lot easier but I don't get how this works.

First off a weak acid/base example
Ka = \frac{(x)(x)}{initial\ amount - x}
if I omit 1 x I get x = \sqrt{Ka(initial\ amount)}
if I omitted 2 x's I'd get x = Ka(initial\ amount) a completely different answer. Does this only apply if I have a (constant \pm x) then I just omit the plus/minus x?

 

[Borek]

x can be omitted only in sums.

There is a rule of thumb (so called 5% rule) which says that a+x can be approximated by a if x is less than 5% of a. See here.

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