Question

Find the sum of a 9-term geometric sequence when the first term is 4 and the last term is 1,024 and select the correct answer below.

Answer 1

first term = a1 = 4, common ratio = r

9th (last) term = a1r^8 = 4*r^8 = 1024

so 4 r^8 / 4 = 1024/4

r^8 = 1024

can you find what r is?

Answer 2

is it 2.4

Answer 3

sorry i made a mistake in my first post

r^8 = 1024/4 = 256

Answer 4

r^8 = 256
r = 2

Answer 5

now we apply the formula for the sum of n terms

Sn = a1 * (r^n - 1
------
r - 1

S9 = 4 * (2^9 - 1)
--------
2 - 1

Answer 6

can you work that out?

Answer 7

S9 = 4 * (512 - 1)

Answer 8

thank you

Answer 9

yw