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Exactly. You should know better than me that "a couple of something" means "a few something", and generally not "two of this something". :-)

X and Y to represent unknowns are a relatively recent invention. Until a couple of centuries ago, algebra was basically written out (like "take a number, add 2 to it and multiply by 3" to mean...

To me it is. :-)

Since he asked, let me extend the question: what about function symbols, such as "sin", "cos", "lim", etc.?

It's not necessary to join them, but it is certainly faster to write joined letters than one by one.

http://masterrussian.com/blalphabet.shtml - Not so useful, since it doesn't show how to join the letters.

http://www.lang.ourfamily.com/propisi/home.html - This might help also.