9 + 10 = 21?

Not quite, but

e^(9+10)πi = e^21πi

So there. (If your household isn't as steeped in the 9+10=21 meme as is mine, coming from my 10- and 11-year-olds and their school chums, see here. And, yes, there are other "proofs" that 9+10=21, too.)

UPDATE: Uh-oh - the superscript HTML tags don't seem to be working, so I switched to using ^ to indicate exponentiation.