Question

2.What is the sum of the geometric sequence 4, 12, 36 … if there are 9 terms?

Answer 1

@texaschic101 i tried to do this one on my own but i am lost

Answer 2

first you have to find the difference....do you know what the difference is ?

Answer 3

yes it is 4

Answer 4

try again

Answer 5

oh sorry it is 3

Answer 6

correct.

now the sum formula is : sn = a1( 1 - r^n) / (1 - r)
n = 9 (because we want total of 9 terms)
r = 3
a1 = 4 (a1 is the first term)

you understand all that so far ?

Answer 7

yes

Answer 8

now lets sub our stuff into the formula...
s9 = 4 ( 1 - 3^9) / (1 - 3)
s9 = 4 (1 - 19683) / (-2)

with me so far ? I had to use an exponent calculator

Answer 9

yes i am following so far

Answer 10

i am wrighting down each step that you are showing me so i can do it on my own

Answer 11

s9 = 4(- 19682) / (-2)
s9 = - 78728 / -2 (remember when dividing by two negatives the answer will be positive)
s9 = 39364

do you have any questions ?

Answer 12

yes do i divide s9 into 39364?

Answer 13

no....s9 is just saying the sum of the 9 terms....your answer is 39364

Answer 14

oh ok i understand

Answer 15

cool. You know I still can't remember this formula, I always have to look it up. I have so many formulas running in my head...lol

Answer 16

yeah i know that feeling wjhy does there have to be so many formulas

Answer 17

I don't know, but there is way toooo many to remember

Answer 18

so true

Answer 19

thanks again:)

Answer 20

anytime :)