How to properly cite a domain-specific result of a generic concept from a paper? I plan to cite two sets of equations (referred to as Equations X and Y) from a paper. Equations X that hold for a model look like textbook results to me, but the authors give a lengthy explanation with a citation to another paper. Equations Y were derived for a special case of the model, since only the special case is useful in that paper. I am working on a different problem and will use this model. I need to include both Equations X and Y in my paper.  

Is it appropriate to cite the Equations X followed by a similar but much shorter explanation as follows:


  The following equations hold for these performance variables, as also seen in [citation]?


Is it enough or should more credits be given? 

For equations X, it is quite obvious that it also holds for the general Model. Can I write something like the following?


  [citation] has showed the above equation for the special case where L=0. It can be easily verified that it also holds for L>0.
To be pedantic, you can say something like "The following equation (eq-A) is from [paper-A], in [paper-B] the special case (eq-B) is derived from the above."

Season to taste.