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

1. Dido525 Group Title

did you mean n=1?

2. Dido525 Group Title

@infinitemoes

3. infinitemoes Group Title

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

4. mrbarry Group Title

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

5. infinitemoes Group Title

yes exactly like that

6. mrbarry Group Title

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

7. infinitemoes Group Title

alright..

8. mrbarry Group Title

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 Group Title

I think I got the answer. -110

10. mrbarry Group Title

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 Group Title

thank you! :)

12. mrbarry Group Title

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

13. infinitemoes Group Title

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

14. mrbarry Group Title

Of course.