Question

“If S = -- [2a + (n − 1)d], find the value of S when n = 42, a = 50, d = −2.”

Can someone please explain, I got the answer from a friend, but still don’t get how to get there (answer is s=378)

Answer 1

see explanation

Explanation:

Note that the formula given is incorrect.

The formula for the sum to n terms of an arithmetic sequence is

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

Substituting the given values into the formula

`S_(42)` =
`(42)/(2)` [ (2 × 50) + ( - 2 × 41) ]

= 21 ( 100 - 82)

= 21 × 18 = 378