I just realized that I am a barbarian when it comes to writing. But I am not entirely sure, so this might be the right place to ask. When typing display-mode formulae do you guys add a period after the formula ends a sentence? 

Like:


  This is the formula for a circle $$x^2 + y^2 = r^2.$$
  Therefore blabla...


or 


  This is the formula for a circle $$x^2 + y^2 = r^2$$
  Therefore blabla...


My supervisor has been complaining a lot that I don't use period and commas in my display-mode formulae. But I get uneasy doing that because it doesn't feel natural to me, I took a look at two books at random and both of them so far do the punctuation in their display formulae.. I know this is stupid of me and its amazing I have never noticed that.

Edit: This would be a fantastic opportunity to see what people actually like as opposed to what they think they like.  Everyone who has an opinion on what the punctuation should be should provide an illustrative example of such so that by the voting it can be seen what is actually preferred.  If you do this, make your answer just the example (so provide any general homilies in another answer) so that the voting truly reflects the community view of the example.

My meta-guide with respect to that is 


  Tautology 2.3.1 — A mathematical text is, before everything else, a text.


from Michèle Audin's Conseils aux auteurs de textes mathématiques, which you can get from her webpage.

A corollary is that when one writes a mathematical text one is writing sentences, to which all rules which apply to sentences of course apply. And, say, sentences end in a period.
