Question

Find Sn for the given arithmetic series.

a1 = –74, d = –3, n = 20

Answer 1

- 1050

Explanation:

The sum to n terms of an arithmetic sequence is

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

`S_(20)` =
`(20)/(2)` [ (2 × - 74) + (19 × - 3) ]

= 10 ( - 148 - 57) = 10 × - 105 = - 1050