Let X and Y be finite sets.
a) Suppose the X C Y and |X| = |Y|. Use 10.2.1 to prove X =Y.
Theorem 10.2.1 (The adddition principle):
Suppose that X and Y are disjoint finite sets. Then X U Y is finite and | X U Y| = |X| + |Y|.
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Set Theory Proofs and the Addition Principle are investigated. The solution is detailed and well presented.