Proposition 10.2.1: (the addition principle)
Suppose that X and Y are disjoint finite sets. Then X U Y is finite and
|X UY| = |X| + |Y|.
Corollary 10.2.2:
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|.
