4. (10 points) Show that if A is an infinite set, then whenever B is a set, AUB is also an infinite set.

Algebra & Trigonometry with Analytic Geometry
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Chapter2: Equations And Inequalities
Section2.6: Inequalities
Problem 80E
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4. (10 points) Show that if A is an infinite set, then whenever B is a set, AUB is also an infinite set.
Solution: If AUB were finite, then it would have n elements for some natural number n. But A
already has more than n elements, because it is infinite, and AUB has all the elements that A has,
so AUB has more than n elements. This contradiction shows that AUB must be infinite.
Transcribed Image Text:4. (10 points) Show that if A is an infinite set, then whenever B is a set, AUB is also an infinite set. Solution: If AUB were finite, then it would have n elements for some natural number n. But A already has more than n elements, because it is infinite, and AUB has all the elements that A has, so AUB has more than n elements. This contradiction shows that AUB must be infinite.
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