Question

GEOMETRIC SEQUENCE HELP: What is the sum of an 8-term geometric sequence if the first term is 10 and the last term is 781,250?

So far I've got to find r: a8=10r^(8-1)
an=10r^7
781250=10r^7

Answer 1

any help is appreciated

Answer 2

log 781250 = 7 * log 10r

7 = log 781250 / log 10r

7 = 5.8927900304 / log 10r

log 10r = 5.8927900304 / 7

log 10r = 0.8418271472

10r = 6.947477472

r = .6947477472

Answer 3

erm although that may be right I thought I was suppose to get r=5 ? without the use of logs

Answer 4

If r = 5 then 10r ^7 ?= 781250

10r = 50

50^7 = 781,250,000,000

5^7 = 78,125

I think that 10r is creating problems

Answer 5

okay well im using this an=a1*r^(n-1) that's were i got everything from im just having trouble finding the final answer of r

Answer 6

what does an mean and also a1?

Answer 7

an number of terms in the sequence and a1 is the first term

Answer 8

Okay how about explaining how you got this?
781250=10r^7
And where does that 10 come from?

Answer 9

What is the sum of an 8-term geometric sequence if the first term is 10 and the last term is 781,250?

Answer 10

10=a1
8=n
781250=an

Answer 11

a₈ = a₁*r^(8 - 1) ==> 781250 = 10r^7
r = (781250/10)^(1/7) = 5.
that's the answer apparently but this is the part I'm not understanding, this isn't my work

Answer 12

Wonder if the formula written like this?
781,250=(10*r)^7

Answer 13

yes