Question

How do you find the sum of a geometric sequence? (-3,18,-108... if there are 8 terms)

Answer 1

First find the common ratio

Answer 2

thats when you deivide the second term by the first, right? so it would be -6?

Answer 3

divide the second term by the first term to find the common ratio

Answer 4

Yes,-6 is correct

Answer 5

okay, then what?

Answer 6

Now, use this

`last term = a (common ratio ^ (number of terms - 1))`

Answer 7

okay, i got 839808 but thats not an answer i have to choose from.

Answer 8

wait

Answer 9

(-3,18,-108... if there are 8 terms)
i think now you should do this
-108 x -6 = 648
648 x -6 = -3888
-3888 x -6 = 23328
23328 x -6 = -139968

Answer 10

so now
-139968 = -3 times - 6 ^n-1

Answer 11

what are your options?

Answer 12

my options are -719835, 719835, 119973, and -119973

Answer 13

I think its 719835

Answer 14

Certainly

Answer 15

Great, thank you

Answer 16

http://hotmath.com/hotmath_help/topics/sum-of-the-first-n-terms-of-a-geometric-sequence.html