The common difference of an A.P., the sum of whose n terms is Sn, is
an is the nth term of an A.P and a n–1 is the (n–1)th term of an A.P,
d = common difference, Sn = sum of n terms of an A.P
d= an – an–1
But an= Sn – Sn–1
And an–1= Sn–1 – Sn–2
So d= Sn – Sn–1 – (Sn–1 – Sn–2)
d= Sn – 2 Sn–1 + Sn–2