Question

in
an AP, of sn =3n2 +5n and ak=164.
Find the Value of K​

Answer 1

Answer:

S

n

=3n

2

+5n

∴S

1

=a=3+5=8

and S

2

=a

1

+a

2

=3×2

2

+5×2=22

⇒a+a+d=22⇒2a+d=22

⇒d=22−2a=22−16=6

Now a

k

=164

⇒a+(k−1)d=164

⇒8+(k−1)6=164

⇒(k−1)6=156

⇒k−1=26⇒k=27