Answer
85
Work Step by Step
First, we use the formula: $(a+b)(a-b)=a^{2}-b^{2}$. Then, knowing that $i=\sqrt{-1}$, we can substitute $-1$ for $i^{2}$ to obtain:
$(2+9i)(2-9i)$
=$(2)^{2}-(9i)^{2}$
=$4-81i^{2}$
=$4-81(-1)$
=$4+81$
=$85$
Elementary Algebra
by Kaufmann, Jerome; Schwitters, Karen;
Published by
Cengage Learning
ISBN 10:
1285194055
ISBN 13:
978-1-28519-405-9
Chapter 11 - Additional Topics - Chapter 11 Review Problem Set - Page 520: 52
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