Question

I am confused with this formula in Arithmetic progression (AKA Series)

State the values of a and in each of the following and find Sn as indicated.

7 + 3 - 1 - 5 - . . ..., S20

Using Sn = N/2 ( 2a +(n-1)d

I know a = 7 d= -4 L= S20

Answer 1

so then plug in your values for n=20

Answer 2

It gives me - 620, I am not sure is right!

Answer 3

Entered as 20/2 ( 2x7 + (20-1) -4 = -620

Answer 4

that's correct. you can check your calculations by manually adding the series, since you know that each term is just the previous term -4.

Answer 5

doing this on a calculator you can see that S(20) is -620

Answer 6

Isn't it just 7-4n?

Answer 7

Ignore me, I haven't woke up yet...

Answer 8

@estudier I did that first but I was told it was the other formula, otherwise the answer would had ben - 69.

Thanks Napervillian!

Answer 9

no prob! the other formula uses the geometric series trick, it's a good one to learn.

Answer 10

nvm. it's not geometric sequence. i've been up way too late tonight.