CAN SOMEONE HELP ME PLEASE!?
what is the sum of the geometric sequence -4,24,-144,...if there are 8 terms?

Question

CAN SOMEONE HELP ME PLEASE!?
what is the sum of the geometric sequence -4,24,-144,...if there are 8 terms?

anonymous · Answer

pull out your trusty calculator

cwrw238 · Answer

sum of n terms of a GS is

Sn  = a1  * ( r^n  - 1)
                 --------
                    r - 1

where a1 = first term, n = number of terms and r = common ratio

cwrw238 · Answer

r can be found by dividing the first term by the second.

anonymous · Answer

why are we using that formula if it's for geometric not arithmetic? lol sorry im a little comfused

cwrw238 · Answer

that formula is for a GS

cwrw238 · Answer

a1 = 4, n = 8 and r = 24 / -4

anonymous · Answer

can you show me how to do it?

cwrw238 · Answer

well plug those values into yhe formula  and use your calculator to work it out

cwrw238 · Answer

first work out (-6)^8 - 1 on your calculator

anonymous · Answer

|dw:1404166199533:dw| did I plug it in right?

anonymous · Answer

|dw:1404166309828:dw| is that right?