Sn is the sum of the first n terms To find the sum of the terms of an arithmetic sequence, you can use the
following formulae:
A. If the first and last terms are
given:

Sn=n(A1+An)÷2

where:
Sn is the sum of the first n terms
A1 is the first term
An is the last terma
B. If the last term is not given:

Sn=n[2a+ d(n-1)]÷2
where:
Sn is the sum of the first n terms
A1 is the first term
d is the common difference

Can you prove these formulae?
One way to prove thee formulae is to use mathematica induction.