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.