Question

Please help! What is the sum of the series? 20 on top and n-1 on the bottom of ∑(5-n)

Answer 1

did you mean n=1?

Answer 2

@infinitemoes

Answer 3

no it's a n-1 on the bottom of the sigma.

Answer 4

\[\sum_{n-1}^{20} (5-n)\]

Answer 5

yes exactly like that

Answer 6

It should be n=1 I am certain.

If we did this "manually" we would have 4 + 3 + 2 + ...

Right?

Answer 7

alright..

Answer 8

This is an arithmetic sequence b/c we are adding -1 over and over. There is a formula to solve this, Arithmetic Series Formula.

Do you know it?

Answer 9

I think I got the answer. -110

Answer 10

Let's see. The formula says: 1/2 the number of items times (first term + last term)

1/2 (20) (4 + -15)

10 (-11)

Good job.

Answer 11

thank you! :)

Answer 12

You are welcome. All done? Thanks for the medal.

Answer 13

Well I do have a couple more questions, but I'm gonna post them separate to this post.

Answer 14

Of course.