Question

If the nth term of an A.P. is (2n + 1), then the sum of its first three terms is
A. 6n + 3
B. 15
C. 12
D. 21

Answer 1

option {a} is correct

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 .