## Algebra 1: Common Core (15th Edition)

$6399$
$79\cdot 81=$ ...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. =$6399$