## infinitemoes 2 years ago Please help! What is the sum of the series? 20 on top and n-1 on the bottom of ∑(5-n)

1. Dido525

did you mean n=1?

2. Dido525

@infinitemoes

3. infinitemoes

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

4. mrbarry

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

5. infinitemoes

yes exactly like that

6. mrbarry

It should be n=1 I am certain. If we did this "manually" we would have 4 + 3 + 2 + ... Right?

7. infinitemoes

alright..

8. mrbarry

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?

9. infinitemoes

I think I got the answer. -110

10. mrbarry

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.

11. infinitemoes

thank you! :)

12. mrbarry

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

13. infinitemoes

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

14. mrbarry

Of course.