Good place to start:

https://www.purplemath.com/modules/series.htm


Sum of n terms of an arithmetic sequence = N/2 * (2A1 + (N-1)D)

where N-> number of elements in the sequence
A1-> first term of seq
D-> common difference between any 2 terms


Other formula variation for sum is:

Sum of AP terms = N/2 * ( A1+An)

where N-> no of terms in sequence
A1-> FIRST TERM OF SEQUENCE
An-> n the term of sequence



The n th element in an AP

An = A1 + (n-1)D (Here lowercase n is the nth element)

eg: 4TH ELEMENT
A4 = A1 + (4-1)D
A4 = A1 + 3D

With these formulas and some practice u can solve most problems in sequences.