Ces 2 types d'exercices peuvent ils être dans le même model ?
I believe so, however I have no experience in this specific domain. Just a suggestion, I assume you already have some way to represent a formula in the database, so perhaps you could have an attribute specifying whether coefficients are fixed, or to be varied. (Or even allow it to be specified per-coefficient.)
So in that case, your model logic for fetching, validation, and so on, would have to take into account which coefficients to assign a varying value (and when).
It seems to me that a view would receive a model object with coefficients already specified, and would have no need to know whether the coefficients are fixed or not, so the view code would not change. Similarly, the controller would depend on the model for initialization before sending to the view, and validation on submission, so could be the same code in either case.
It's also possible to consider using inheritance, a base class for the common logic, and inherited classes with specific overrides. RoR has facilities for dealing with this. I personally don't think it's needed here; I mention it only because I don't know the details of your model representation, so it's possible that inheritance is more appropriate to your needs than I realize.
Suis je clair ?
I think so. It seems to me your problem is more with representation of the underlying model, and that any RoR questions you may have other than creating the mode will be relatively simple.