Question

The sum of first n terms of an AP is 5n^2+3n. If it's n^th term is 168, find the value of n.Also, find the 20 ^th term of this AP.

Answer 1

given ,
Sn=5n^2+3n

we know ,
Tn = Sn -S (n-1)
=5n^2+3n-5(n-1)^2-3 (n-1)
= 5 (n^2-n^2+2n-1)+3 (n-n+1)
=10n -5+3
=10n -2

a/c to question ,
Tn=168
10n-2=168
10n =170
n=17

again ,
T20=10 (20)-2=200-2 =198