arithmatic and geometric sequences

[BigGlenntheHeavy]

\(\displaystyle Ok, \ n \ = \ 9, \ a_1 \ = \ 100, \ and \ l \ = \ 300.\)

 

[Deleted member 4993]

I thought I posted my second question some time ago, but I guess not. I reviewed the answer to the previous question and I was wondering how you all came up with the 400 being divided by 2. Where did the 2 come from and how did you add an extra 100 to the 300?

That is the equation for summation of arithmatic sequence.

\(\displaystyle a_1\ + \ a_2 \ + \ a_3 \ + .... + \ a_n \ = \ \frac{n \ * \ (a_1 \ + \ a_n)}{2}\)

If you are interested in finding the derivation of this equation - do a google search and tell us what you found. (Your textbook should have this equation too)