Arithmetic progression

In mathematics, an arithmetic progression, or arithmetic sequence, is a sequence. The difference between each two terms which follow one another is always the same. This is different from a geometric progression, where the ratio of change between two terms is constant.
If the initial term of an arithmetic progression is $a_1$ and the common difference is $d$, then the $n$-th term of the sequence ($a_n$) is given by:

$a_n=a_1+(n-1)d$

A finite portion of an arithmetic progression is called a finite arithmetic progression and sometimes just called an arithmetic progression. The sum of a finite arithmetic progression is called an arithmetic series. Template:Math-stub