Answer
$a^2+a-90$
Work Step by Step
Using $(a+b)(c+d)=ac+(ad+bc)+bd$, or the FOIL Method, the given expression, $
(a+10)(a-9)
,$ is equivalent to
\begin{array}{l}\require{cancel}
a^2+(-9a+10a)-90
\\\\=
a^2+a-90
.\end{array}
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