Proposition 10.2.1: (the addition principle)
Suppose that X and Y are disjoint finite sets. Then X U Y is finite and |X U Y| = |X| + |Y|.
For a positive integer n, suppose that X1, X2....,Xn is a collection of n pairwise disjoint finite sets (i.e. i does not = j => Xi Xj = empty set)
Then X1 U X2 U....U Xn = U ( lim from n to i=1) Xi is a finite set and
|X1 U X2 U......U Xn| = |X1| + |X2|+....|Xn|.
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Addition principle, induction, union and pairwise disjoint finite sets are investigated and discussed. The solution is detailed and well presented.