Answer
$4005$
Work Step by Step
${n \choose k}=\frac{n!}{k!(n-k)!}$ Hence here: ${90\choose 2}=\frac{90!}{2!(90-2)!}=\frac{90\cdot89\cdot88!}{2!88!}=\frac{90\cdot89}{2}=\frac{8010}{2}=4005$
College Algebra (6th Edition)
by Blitzer, Robert F.
Published by
Pearson
ISBN 10:
0-32178-228-3
ISBN 13:
978-0-32178-228-1
Chapter 8 - Summary, Review, and Test - Review Exercises - Page 790: 63
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