Arithmetic progression

Arithmetic sequence is a sequence of numbers such that the difference between the consecutive terms is constant.
For example: 1, 3, 5,.....,11,13,15,...

a1,a2,a3,...,an1,an,an+1,...

n-th term of the sequence:

an=a1+(n1)d
an=an1+an+12,  n>1

The sum of the first n terms:

Sn=a1+an·n2
Sn=[2a1+(n1)d]·n2
Keywords: Arithmetic progression