Answer
We want the 4th term, and when we have a binomial expansion, we want the $(r+1)^{th}$ term.
Work Step by Step
Since $r+1=4$, we see that $r=3$, so that’s why we use $r=3$.
Intermediate Algebra (6th Edition)
by Martin-Gay, Elayn
Published by
Pearson
ISBN 10:
0321785045
ISBN 13:
978-0-32178-504-6
Chapter 11 - Section 11.5 - The Binomial Theorem - Vocabulary, Readiness & Video Check - Page 663: 10
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