The sum of n terms of an A.P is given by S_n

S_n = (2a + (n–1) d)

If the sum of n terms is given that is S_n is given then the n^th term is given by the formula

T_n = S_n– S_n–1

Where S_n–1 is sum of the (n–1)^th term of the A.P.