Question 429014
A sequence is a collection of terms or objects in some order. For example, {4,7,10,13,...}, {6.4, pi, 8, -5,...} and {cat,dog,chicken,horse,...} can be considered sequences. A series is defined as the sum of the terms in a sequence.

For those three questions, use the formula {{{S[n] = (n/2)(a[1] + a[n])}}} where n is the number of terms in the arithmetic sequence, {{{S[n]}}} is the sum of the first n terms, {{{a[1]}}} is the first term, and {{{a[n]}}} is the n-th term. I rarely see students try to memorize the formula, since it is often easier to re-derive it instead of memorize it (being able to derive the formula is also more advantageous than memorizing).