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Challenge: (Lemma 4.2) Let / and J be open intervals, a e I, be J and f : I x J \\ {(a, b)} - R a function such that lim (r,y) + (a,b) f(r, y) exists and lim f(r, y) exists for all re I \\ {a}. Prove that y-+b lim f(r, y) = lim (r,y) +(a,b) lim f(x, y) ) y-+6