Answer
$n(A)+n(B)-n(A\cap B)$
Work Step by Step
It is the number of elements in A plus the number of elements in B, minus the number of elements in their intersection (because we counted those twice).
Thus,
$n(A\cup B)=n(A)+n(B)-n(A\cap B)$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 13 - Counting and Probability - 13.1 Counting - 13.1 Asses Your Understanding - Page 847: 7
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