列出计算等差数列之和的公式。 将变量n{\displaystyle n}、a1{\displaystyle a_{1}}和an{\displaystyle a_{n}}代入公式中。 计算首项和末项的平均数。 用平均数乘以数列的项数。

I'm interested in that, but I only have a few mins. Can you give me at most the first 500 characters of a detailed explanation in English about that?
The formula is Sn=n(a1+an2){\displaystyle S_{n}=n({\frac {a_{1}+a_{n}}{2}})}, where Sn{\displaystyle S_{n}} equals the sum of the sequence. Note that this formula is indicating that the sum of the arithmetic sequence is equal to the average of the first and last term, multiplied by the number of terms. Make sure you make the correct substitutions. For example, if you have 5 terms in your sequence, and 10 is the first term, and 30 is the last term, your formula will look like this: Sn=5(10+302){\