Chapter 7 - Section 7.4 - The Binomial Theorem - 7.4 Exercises - Page 664: 19

$12$

Using $_nC_r=\dfrac{n!}{r!(n-r)!},$ the given expression, $ _{12}C_{1} $ evaluates to \begin{array}{l}\require{cancel} =\dfrac{12!}{1!(12-1)!} \\\\= \dfrac{12!}{1!11!} \\\\= \dfrac{12(11!)}{(1)(11!)} \\\\= \dfrac{12(\cancel{11!})}{(1)(\cancel{11!})} \\\\= \dfrac{12}{1} \\\\= 12 \end{array}