Question

How do you find the sum of the arithmetic sequence given a1 = 5, d = 7, n = 120?

Answer 1

`color(blue)(= 50580`

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 = 5, d=7 and n=120`

`= (120/2)[2 . 5 + (120 - 1) . 7]`

`= 60 . [10+ 119 . (7)]`
`= 60 . [10 + 833]`
`= 60 . (843)`
`color(blue)(= 50580`