Answer
$159,999$
Work Step by Step
$399\cdot 401=$
...write as a product of a sum and a difference.
$=(400+1)(400-1)$
...use $(a+b)(a-b)=a^{2}-b^{2}$.
$=400^{2}-1^{2}$
...simplify powers.
$=160,000-1$
...simplify.
$=159,999$
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