Arithmetic progression (AP)
General term or nth term
Tn = a + (n - 1) d where a is the first term and d is the common difference.
Some of n terms
Sn = n/2 [2a + (n - 1) d] where a is the first term and d is the common difference.
Or
Sn = n/2 (a + l) where l is the last term.