Answer
$a^2-2ar+r^2$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
(a-r)^2
,$ simplifies to
\begin{array}{l}\require{cancel}
(a)^2+2(a)(-r)+(-r)^2
\\\\=
a^2-2ar+r^2
.\end{array}
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