Answer
$5$
Work Step by Step
Use special formula $(a+b)(a-b)=a^2-b^2$.
We have $a=2$ and $b=i$.
$(2+i)(2-i)\\
=2^2-i^2\\
=4-i^2$
Use $i^2=-1$
$=4-(-1)$
$=4+1$
$=5$
Hence, the answer is $5$.
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Appendix A - Review - A.7 Complex Numbers; Quadratic Equations in the Complex Number System - A.7 Assess Your Understanding - Page A60: 2
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