Is it? I didn't bother to check as it wasn't too relevant. I wasn't stuck anymore. Worst-case it's a memnonic that is untrue. Best case: it is. For actual derivations I check the work.

This is what it wrote by the way.

I posted an image and asked:

Why is this m

m for angle mangle?

ChatGPT:

In geometry we often need to distinguish the angle itself (a geometric object) from the numerical size of that angle (a real-number measurement, usually in degrees or radians). To keep the two ideas separate we put a little “m” in front of the angle symbol:

m∠B reads as “the measure of angle B.”

* ∠B refers to the actual wedge-shaped region determined by the two rays that meet at vertex .

* m∠B is the single number—e.g. \90∘—that tells you how “wide” that wedge is.

So in the worked example the author writes

m∠A + m∠B + m∠C + m∠D = 360∘

because they are adding the sizes of the four interior angles of a quadrilateral, not the angles themselves.

If you dropped the m you’d be adding geometric objects, which doesn’t make sense; the m reminds the reader that what’s being added are real-number measures. (Some textbooks use ∣∠B∣ or simply write the letter with a bar over it, but “m∠” is by far the most common in U.S. and many international curricula.)

So the mysterious “m” isn’t another variable—just a shorthand for “measure of.”

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Edit: this took quite some work to write well as ChatGPT answers don't copy/paste neatly into HN. So I won't be doing this for the other example.