Question

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

Answer 1

`=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)`

`= 20 /2 (81 - 13)`

`= 10 xx 68`

`=color(blue)(680`