Answer
$-13$
Work Step by Step
The first two terms of $
a_n=n(n-6)
$ are
\begin{array}{l}
a_1=1(1-6)\\
a_1=1(-5)\\
a_1=-5
,\\\\
a_2=2(2-6)\\
a_2=2(-4)\\
a_2=-8
.\end{array}
\\\text{
Hence, the partial sum is $
-5+(-8)=
-13
$.
}
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