**eMathematician Professional**

General rule of arithmetic sequence can be written by this formula, where [tex]a_1[/tex] - the first term, [tex]d[/tex] - the difference between the terms, [tex]n[/tex] - the number of term. $$a_n=a_1+(n-1)d$$

To sum up the terms of an arithmetic sequence we use this formula: $$S_n=\frac{2a_1+(n-1)d}{2}n$$