Question

If the sum of first n terms of an A.P. is given by Sn= 3n2+5n, find the nth term of the A.P.

Answer 1

Sn=3n²+5n

S₁=3(1)²+5(1)

=3+5

=8............= a

S₂=3(2)²+5(2)

=12+10

=22

so on subtracting S₂-S₁=A₂

22-8

=14

So∴

a=8

d=A₂-A₁

=14-8

=6

So nth term is

An=a+(n-1)d

=8+(n-1)6

=8+6n-6

=2+6n

Answer 2

Answer:

sn1=8

sn2=22

a1+a2=sn2

sn2-a1=a2

a2=14

a2-a1=6

an =2+6n