Question

The sum of the first n terms of an AP is 2n2+5n, then its n

th term is​

Answer 1

Step-by-step explanation:

Given: Sn = 2n²+5n.

To find: an=?

If n=1,

S1 or a = 2×1²+5×1

=> 2+5

a = 7.

If n=2,

S2 =2×2²+5×2

=> 2×4+5×2

=> 8+10

=> 18.

S2= a+A2.

A2=S2-a

=> 18-7

A2 => 11.

Common difference, c.d=a2-a

=> 11-7.

d = 4.

Formula of nth term of A.P is :

$an = a + (n - 1) \times d$

=> An = 7+(n-1)×4

=> 7+4n-4

=> 4n-3.