# How do you find the sum of the arithmetic sequence given a1 = 81, an = - 13, n = 20?

How do you find the sum of the arithmetic sequence given a1 = 81, an = - 13, n = 20?
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Ashlee Ramos
The formula for sum of an Arithmetic Sequence when the first term ${a}_{1}$, number of terms n and last term ${a}_{n}$ are given is:
$Sum=\left(\frac{n}{2}\right)\left({a}_{1}+{a}_{n}\right)$
$=\frac{20}{2}\left(81-13\right)$
$=10×68$
$=680$