Thank you for the opportunity to help you with your question!

Suppose a
sequence
of numbers is arithmetic (that is, it increases or decreases by a constant amount each term), and you want to find the sum of the first n terms.

Denote this partial sum by S_{n}. Then

where n is the number of terms, a_{1} is the first term and a_{n} is the last term.

Please let me know if you need any clarification. I'm always happy to answer your questions.