Question

How do you find the sum of the arithmetic sequence given a1 = 7, d = - 3, n = 20?

Answer 1

`color(blue)(= -430`

Explanation:

The formula for the sum of an arithmetic sequence when `a, d and n` are given is:

`color(blue)(Sum = (n/2)[2a + (n-1)d]`

Here :`a1 = 7, d=-3 and n=20`

`= (20/2)[2 . 7 + (20 - 1) . -3]`
`= 10 . [14+ 19 .( -3)]`
`= 10 . [14 -57]`
`= 10 . (-43)`
`color(blue)(= -430`