what is the sum of the geometric sequence -4, 12, -36 if there are 7 terms

Question

what is the sum of the geometric sequence  -4, 12, -36 if there are 7 terms

ghazi · Answer

s= n/2(2*a+(n-1)d) apply this

anonymous · Answer

i dont understand how to apply can u show me in steps?
:

jim_thompson5910 · Answer

wrong formula ghazi, that formula applies to arithmetic sequences

anonymous · Answer

i need geometric sequences

cwrw238 · Answer

sum of n terms  = a * r^n - 1
                                  ------
                                   r - 1

where a = first term and r = common ratio
common ratio for this series is 12 / -4 = -3
and a = -4

ghazi · Answer

oh yea....sorry that was a mistake

anonymous · Answer

if the 7th term is in a geometric sequence is 64  and the common ratio is -2 what are the 1st and 14th term ?

amistre64 · Answer

please post new questions to the left after closing old ones.

cwrw238 · Answer

n = 7

sum 7 terms =  -4 * (-3)^2  - 1
                                ---------
                                    -3-1

cwrw238 · Answer

sorry - replace (-3)^2  with (-3)^7       (n=7)

anonymous · Answer

so the answere is n=7
?

cwrw238 · Answer

no  - 7 is number of terms 

sum = (-4) * (-3)^7  - 1
                    ---------
                          (-4)

- calculator job

anonymous · Answer

so what would be the answere?

cwrw238 · Answer

note (-4) cancel  out so its

 (-3)^7 - 1
- you can do this on a calculator

cwrw238 · Answer

- can't you?

anonymous · Answer

-21888

anonymous · Answer

right?

anonymous · Answer

???

cwrw238 · Answer

yes

anonymous · Answer

thankuu so much! tehre is one more question i have if that wouldnt bother u to help please??