#### Answer

$6399$

#### Work Step by Step

$79\cdot 81=$
...write as a product of a sum and a difference.
$=(80+1)(80-1)$
...use $(a+b)(a-b)=a^{2}-b^{2}$.
$=80^{2}-1^{2}$
...simplify powers.
$=6400-1$
...simplify.
=$6399$

Published by
Prentice Hall

ISBN 10:
0133281140

ISBN 13:
978-0-13328-114-9

$6399$

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