After reading the links provided by the mods in a question I recently posted with an answer, I believe that my question conforms in spirit the criteria that the moderators have tried to infuse in our site through their work.
While answering own questions is explicitly encouraged in the link right next to the box saying "answer your own question," some moderators seem concerned that naughty users may post solutions to the entire e.g., Sears & Zemansky book, using this format. And the general feeling is that people can post exercises and problems with educational purpose as long as: (i) the problem is not entirely trivial and (ii) who posts them also shows their work toward the solution.
Well, my question is not completely trivial: I did not know the result before thinking a lot while solving a similar problem. It is not from any book I know of. I agree the formulation is terse, but clear, and I put a lot of work in the answer of the question. I posted it just to share the effort and result. People is free to downvote it to oblivion if they do not like it, but closing it is something else: it is a penalty for something I did wrong.