What i do, is that i keep a latex document open at all times while doing
mathematical notes.

then what i do is if i need to write down a formula i write it in tex,
compile it, and insert as "screen clip" (dont know if that's the english
word, i use the danish version and that's the best translation of the
function i could come up with, but i guess you understand what i mean) in my
onenote