i did that, i did the philosophical "divide the interval to 10 pieces infinite times" one, and i did one with the limit of 1/10n when n goes to infinity, which is the one that is actually legit undisputable, unless someone doesnt believe in supremums and series in general.
also if one doesnt believe in algebra, how am i supposed to prove anything in maths to him? unless is show him proofs for everything in algebra, which he can again deny, like he denies the algebraic proof of .9999 = 1....