Answer
$S_{10}=1090$
Work Step by Step
RECALL:
The sum of the first $n$ terms ($S_n$) of an arithmetic sequence is given by the formula:
$S_n=\dfrac{n}{2}\left[2a+(n-1)d\right]$
where
$a$ = first term
$d$ = common difference
The given arithmetic sequence has:
$a=55
\\d=12$
Thus, to find the sum of the first 10 terms, substitute the given values to the formula above to obtain:
$S_n=\dfrac{n}{2}[2a+(n-1)d]
\\S_{10}=\dfrac{10}{2}[2(55) + (10-1)(12)]
\\S_{10}=5[110+9(12)]
\\S_{10}=5(110+108)
\\S_{10}=5(218)
\\S_{10}=1090$
