1. Sum of first n terms of an A.P.
is
S n = (n/2)[2a +(n -1)d]
2. If Sn Is sum of n terms of an
A.P. whose first term is a and
last term is l,
then S n = (n/2)(a + l)
3. If common difference is d,
number of terms n and the
last term l,
then S n = (n/2)[2l-(n -1)d]