Question

if the sum of n terms of an ap is 2n square + 5 n then find the 4th term​

Answer 1

Let the sum of n terms of A.P = Sn

given, Sn = 2n square + 5 n

Now, (n )th term of A.P= Sn- Sn-1

(or) a(n) = ( 2n square + 5n ) - ( 2 ( n-1)^2 + 5 ( n-1) ).

2n square + 5n - ( 2n square - 4n + 2+ 5n -5 )

= 2n square +5n - 2n square- n+3

a(n) = 4n+3

4th term (a4) = 4×4+3= 19