Answer
$84$
Work Step by Step
According to the Binomial Theorem, the term containing $x^j$ in the expansion of $(ax+b)^n$ is given by:
${n\choose n-j}a^jb^{n-j}x^j.$
Hence, the coefficient of $x^2$ is
${7\choose 7-2}2^2(1)^{7-2}=84$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 12 - Sequences; Induction; the Binomial Theorem - Chapter Review - Review Exercises - Page 839: 33
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