What is Arithmetic Progression?

The $n$th term is $a_n = a + (n-1)d$, where $a$ is the first term and $d$ is the common difference. The sum of the first $n$ terms is $S_n = \dfrac{n}{2}\left[2a + (n-1)d\right]$, or $\dfrac{n}{2}(a + l)$ when the last term $l$ is known. If $d$ is positive the terms increase, and if $d$ is negative they decrease.