Variables

a1
First term
an
N-th term
d
Difference between two terms
Sn
Sum of first n terms
1.
n-th terem of an arithmetic sequence
$a_n = a_1 + (n-1)\cdot d$
2.
Sum of the first n terms
$S_n=\dfrac{n}{2}\left[2a_1 + (n-1)\cdot d\right]$
3.
Sum of first n terms
$S_n= \dfrac{n}{2}\left(a_1+a_n\right)$
4.
Difference of arithmetic series
$d=a_n-a_{n-1}$
5.
n-th term
$a_n=\dfrac{a_{n-1}+a_{n+1}}{2}$