정답
$a_n = a_1 + (n - 1)d$, where $a_1$ is the first term and $d$ is the common difference.
해설
The sum of the first $n$ terms is $S_n = \frac{n}{2}(a_1 + a_n)$.
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$a_n = a_1 + (n - 1)d$, where $a_1$ is the first term and $d$ is the common difference.
The sum of the first $n$ terms is $S_n = \frac{n}{2}(a_1 + a_n)$.