Question

If the sum of first n terms of A.P.is given by Sn=3n squer+ 5n,find the nth term of the A.P.

Answer 1

put value of n = 1 then
S1 = 3+5 = 8
now put n= 2 then
S2= 3*2^2+ 5*2 = 22
now put n = 3
S3= 3*3^2+5*3 ,= 42
S1= A1
A2 = s2- S1 = 22-8= 14
A3 = s3-s2 = 42-22= 20
so series is ...8,14,20
where a= 8
d = 14-8=6
a9= a+8d = 8+8*6 = 56