An arithmetic sequence is a sequence of numbers in which the difference between consecutive terms is constant. This difference is known as the common difference.
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The general form of an arithmetic sequence is $a_n = a_1 + (n-1)d$, where $a_1$ is the first term, $d$ is the common difference, and $n$ is the term number.
The sum of the first $n$ terms of an arithmetic sequence (arithmetic series) can be calculated using $S_n = \frac{n}{2} \times (2a_1 + (n-1)d)$ or $S_n = \frac{n}{2} \times (a_1 + a_n)$.
In any arithmetic sequence, the nth term can be found if you know any three of these values: first term ($a_1$), common difference ($d$), number of terms ($n$), and nth term ($a_n$).
Arithmetic sequences can be either increasing (if $d > 0$) or decreasing (if $d < 0$).
If all terms in an arithmetic sequence are added together, it forms an arithmetic series.