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

Jun 24, 2015

 =color(blue)(680

#### Explanation:

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:

 color(blue)(Sum = (n/2)(a_1+a_n)

$= \frac{20}{2} \left(81 - 13\right)$

$= 10 \times 68$

 =color(blue)(680