I'm trying to write my first paper, and as part of a proof of my conjecture I rely on the fact that the sum of the exterior angles of a simple polygon must equal $2\pi$.
Every high school student is taught this, and it says it here on Wikipedia.
However my professor suggested that I either reference the theorem by name or cite it from a reputable source.
I have searched the web but was unable to find a source like a published paper probing this fact, nor was I able to find a name for this. All I could find were math tutorials explaining it. I could try to prove it myself, but I don't want to make my paper like than it needs to be with trivial mathematics.
Where can I find a good, citable source for this fact?