Question

What is the sum of a 17-term arithmetic sequence where the first term is 6 and the last term is -90?

Answer 1

use the formula

S = n*(a1 + an)/2

where,

S = sum
n = number of terms
a1 = first term
an = nth term

Answer 2

-714 thanks!!! :)

Answer 3

to get a feel for the formula; consider a more do able set of numbers like:

2+3+4+5+6 , there are 5 numbers entotal

if we double this amount and use it in a convenient manner we can work out any arithmetic sum in general; lets reverse the order and add it from underneath

2+3+4+5+6
6+5+4+3+2
------------
8+8+8+8+8

notice that each term is equal to the sum of the first and last term:

we started with 5 numbers: 5(8) = 5(2+6) = 40

but we doubled the value to begin with so we want to halve it to correct it back

40/2 = 20

in general, we can add the first and last terms; multiply it by how many terms there are, and divide the results in half

Answer 4

-714 is correct

Answer 5

Thank you guys! :)

Answer 6

yw

Answer 7

good luck