# Arithmetic sequences and sums

General rule of arithmetic sequence can be written by this formula, where $a_1$ - the first term, $d$ - the difference between the terms, $n$ - the number of term. $$a_n=a_1+(n-1)d$$
To sum up the terms of an arithmetic sequence we use this formula: $$S_n=\frac{2a_1+(n-1)d}{2}n$$

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