Question

plz help urgent
What is the sum of the geometric sequence 8, –16, 32 … if there are 15 terms?

Answer 1

@JA1

Answer 2

@Jasmineflvs

Answer 3

what is the common difference ? Can you answer that ?

Answer 4

8

Answer 5

no...this is geometric not arithmetic.....what does 8 multiply by to = -16, and what does -16 multiply by to equal 32 ?

Answer 6

oh ok that would be -2

Answer 7

i dont understand on how to solve it i have reveowed all of my notes but i am completely lost

Answer 8

i think i just figured it out do i just keep multiplying by -2 until i get to the 15th term?

Answer 9

correct. Now we will use the sum formula...
sn = a1 (1 - r^n) / (1 - r)
n = 15
r = -2
a1 = 8

s15 = 8(1 - (-2^15) / (1 - (-2)
s15 = 8 (1 + 2^15) / 3
s15 = 8(32769)/3
s15 = 262152/3
s15 = 87384

Answer 10

oh ok i see what to do now my teacher wont help me i apperciate your help and patience with me.

Answer 11

you could just keep multiplying by -2 and when you get to the 15th term add everything that you have...which would take awhile

Answer 12

This might help you...
http://www.regentsprep.org/Regents/math/algtrig/ATP2/GeoSeq.htm

Answer 13

just remember, when it is geometric, the common difference is gonna be result of multiplying, but in an arithmetic sequence, the common difference will be result of adding

Answer 14

any more questions ?

Answer 15

not right at this moment i dont but if i do i will definitrly ask you.@texaschic101

Answer 16

I will be happy to help, if I can :)