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3. Let X and Y be countably infinite sets. (a) Prove: If X and Y are disjoint then XuY is countably infinite. (b) Is the stat

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(9) lexx and i are disjoint Bence x and y ane Gountable then Welan write Xz2 a, az, az... } 1 Y=231, 32, 63 ... } Define folNt ęs a subset of Y. So as is either finite of contable. If Az istinite Then AUA, is the Union of an Corntalle set and a finit

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