Question

Find the sum of first 40 terms of 11+14+17+...​

Answer 1

S₄₀ = 2780

Explanation:

there is a common difference between consecutive terms in the sequence, that is

14 - 11 = 17 - 14 = 3

this indicates the sequence is arithmetic with sum to n terms

`S_(n)` =
`(n)/(2)` [ 2a₁ + (n - 1)d ]

where a₁ is the first term and d the common difference

here a₁ = 11 and d = 3 , then

S₄₀ =
`(40)/(2)` [ (2 × 11) + (39 × 3) ]

= 20( 22 + 117)

= 20 × 139

= 2780