Question

Please help. what is the sum of the arithmetic series below

Answer 1

Option A) 585

Explanation:

a1 = 5(1)-1 = 4

a2 = 5(2)-1 = 9

d = a2 - a1 = 9 - 4 = 5

n = 15

`(n)/(2)(2a+(n-1)d)`

=
`(15)/(2)(2(4)+(15-1)5)`

=
`(15)/(2)(8+(14)5)\\= (15)/(2)(8+70)\\= 15 × 39\\= 585`