Precalculus (6th Edition)

Published by Pearson
ISBN 10: 013421742X
ISBN 13: 978-0-13421-742-0

Chapter 11 - Further Topics in Algebra - 11.4 The Binomial Theorem - 11.4 Exercises - Page 1045: 22


$\dfrac {n\left( n-1\right) }{2}$

Work Step by Step

$\binom{n}{n-2}=\dfrac {n!}{\left( n-2\right) !\times \left( n-\left( n-2\right) \right) !}=\dfrac {n!}{\left( n-2\right) !\times 2!}=\dfrac {\left( n-2\right) !\times \left( n-1\right) \times n}{\left( n-2\right) !\times 1\times 2}=\dfrac {n\left( n-1\right) }{2}$
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