Solving for x in calculating pH questions

[Miya Eberlein 1J]

Why do we sometimes ignore the x in the denominator of the ratio of concentrations and sometimes we keep it there, for when we are solving for x in the questions that ask to find the pH?

[Neel Patel - 2A]

If the Ka or the Kb, depending what type of reaction you are working on is really small then the x on the bottom of the denominator is generally ignored because when rounded the final answer is not really affected by the change in x. The x is going to be so small that the equilibrium concentration is not going to be affected.

[Roy Hsieh 1C]

X can be calculated regardless, but in cases where it's so small, it can be ignored to save time. I personally would try to solve as if x is there, but that is only if I have the time to try.