When reading a math proof it helps to spell out what the symbols stand for, instead of saying the symbol names, for example, instead of "E equals m c squared" say "energy equals mass times speed of light squared". This also greatly helps understanding. If you can't keep track because there are too many symbols, make a little lookup table.

You can point to "sin", but in practice math just doesn't use multi-char variable names. The trig functions are special cases. Doesn't mean you're allowed to name arbitrary vars like that.

What about ker, coker, im, hom, end, aut, lim, colim, Set, Top, Man, Mat, Vect, GL, SL, SO, Lie, Gr, Tor, Ext, tr, Rep, char, rank, Isom, ann, hull, Diff, ...? I'm just copying multiletter names out of my preamble.tex file here.

I get that these aren't variables, but it's really not so uncommon outside of trigonometry to have multi-letter names for things that appear in equations.