Question

If the nth term of an ap is 2n+1 then find the sum of its first three terms

Answer 1

T1=2 (1)+1
T1=3

T2=2 (2)+1
T2=5

T3=2 (3)+1
T3=7

S3=T1+T2+T3
S3=3+5+7
S3=15

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 .