Question

if 9th term of an A.P is (2n+1)what is the sum of its three term

Answer 1

a9-2n+1
a+8d=2n+1
a=2n+1-8d
s3=n/2(2a+(n-1)d)
=n/2(2*(2n+1-8d)+dn-d)
n/2(4n+2-16d+dn-d)
n/2(4n+2-17d+dn)
Answer..

Answer 2

Answer:

Given :

nth term = 2 n + 1

= > a₁ = 2 + 1

=  3

= > a₂ = 4 + 1

= 5

d = a₂ - a₁

= > 5 - 3

= 2

Sₙ = n / 2 [ 2 a + ( n - 1 ) d ]

S₃ = 3 / 2 [ 6 + 2 × 2 ]

S₃ = 3 / 2 [ 10 ]

S₃ = 3 × 5

S₃ = 15 .

Therefore , sum of first three numbers is 15 .