Is applying a formula a kind of reasoning? Supposed I have a set of simple rules, e.g. 

if A is between 10 and 20 and B is between 3 and 5, then use 3


and this rules are used in some kind of (more or less) mathematical formula (for example, A + B = C), would you count this as a form of reasoning or inference of a knowledge based system?

You have a knowledge base (the rules) and you get a result (a decision) by applying this rules in a specific way.

I'm curious about your opinions.
Thanks!
You describe what is called expert system in AI. As such, they are artifacts of articial intelligence research.

As far as the Wikipedia definition goes,


  a reasoning system is a software system that generates conclusions from available knowledge using logical techniques such as deduction and induction,


which is vague enough to cover expert systems.

Is there any "intelligence" or independent reasoning in these systems? Certainly not; they are "stupid" in the sense that they just look up values in a table; no learning happens.

You will have to decide which frame of terminolgy reference you want to apply: academic AI or common sense?