Question

Find the sum of the geometric series by using formula.

1-3+9-27+81-243+729-2187

Answer 1

@jim_thompson5910 can you help

Answer 2

what's the first term

Answer 3

the first term is 1

Answer 4

what's the common ratio

Answer 5

Common ratio is -3

Answer 6

how many terms are we summing

Answer 7

or adding up

Answer 8

8

Answer 9

so

a = 1
r = -3
n = 8

Answer 10

Sn = a*(1-r^n)/(1-r)


S8 = 1*(1-(-3)^8)/(1-(-3))


S8 = 1*(1-(-3)^8)/(4)


S8 = 1*(1-6561)/(4)


S8 = 1*(-6560)/(4)


S8 = (-6560)/(4)


S8 = -1640

Answer 11

S8 = sum of first 8 terms

so the sum of the first 8 terms is -1640

sure enough, you can add up 1-3+9-27+81-243+729-2187 to get -1640 and that confirms the answer

Answer 12

okay I got it. Not sure why i thought n was 9. so I was getting the wrong #.

Answer 13

ah i see

Answer 14

thank you!!

Answer 15

you're welcome