Let S be an infinite set and S1, S2, S3, ..., Snbe sets such that S1&c...
To prove that the union of S1, S2, S3, ..., Sn is a subset of S, we can use mathematical induction.
Base Case: For n = 1, we have S1 ⊆ S by assumption.
Inductive Step: Assume that the union of S1, S2, S3, ..., Sn is a subset of S. We need to show that the union of S1, S2, S3, ..., Sn, Sn+1 is also a subset of S.
By definition, the union of S1, S2, S3, ..., Sn, Sn+1 is the set of all elements that are in at least one of these sets. Let x be an arbitrary element in the union of S1, S2, S3, ..., Sn, Sn+1. This means that x is in at least one of these sets.
Case 1: x is in Sn+1. Since Sn+1 ⊆ S by assumption, x is also in S.
Case 2: x is not in Sn+1. In this case, x must be in the union of S1, S2, S3, ..., Sn. By the induction hypothesis, x is in S.
In both cases, we see that x is in S. Since x was an arbitrary element in the union of S1, S2, S3, ..., Sn, Sn+1, we can conclude that the union of S1, S2, S3, ..., Sn, Sn+1 is a subset of S.
Therefore, by mathematical induction, the union of S1, S2, S3, ..., Sn is a subset of S for any positive integer n.