Answer
$a^2-81=(a+9)(a-9)$
Work Step by Step
Remember that the product of the sum and the difference of the same two terms is the difference of their squares.
Since $\sqrt a^2=a$ and $\sqrt {81}=9$
$a^2-81=(a+9)(a-9)$
Algebra 1
by Hall, Prentice
Published by
Prentice Hall
ISBN 10:
0133500403
ISBN 13:
978-0-13350-040-0
Chapter 10 - Radical Expressions and Equations - 10-2 Simplifying Radicals - Mixed Review - Page 612: 87
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