Question

Find the sum of the first 10 terms of the sequence whose a1 = 9, and d = 7.

Answer 1

S₁₀ = 405

Explanation:

the sum to n terms of an arithmetic sequence is

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

a₁ is the first term, d the common difference , n the term number

here a₁ = 9 , d = 7 and n = 10 , then

S₁₀ =
`(10)/(2)` [ (2 × 9) + (9 × 7) ]

= 5( 18 + 63 )

= 5 × 81

= 405