Question

How do i find the number of terms in geometric sequence: 2/81, 4/27, 8/9, ..., 6912?

Answer 1

a = 2/81 or 0.025
r= 10/81 or .123

tn = ar^n-1
6912 = 0.025 (.123)^n-1
----- -----
0.025 0.025
276480 = .123^n-1

Answer 2

I'm stuck. Many have used the "log" but when do I use that? It doesn't seem to work for this problem..

Answer 3

8

Answer 4

\[a_n=\frac{2^n3^{n-1}}{81}\]

Answer 5

how did you get that? o.O

Answer 6

\[a_8=6912\]

Answer 7

a= 2/81 and r = 6
so solve (2/81)(6)^n-1 = 6912

Answer 8

r = 6? o.o

Answer 9

no.. it can't be.

Answer 10

6^n-1 = 6^7
so n=8

Answer 11

get r by dividing the terms

Answer 12

\[a_n=\frac{2^n3^{n-1}}{81}=\frac{2\cdot2^{n-1}3^{n-1}}{81}=\frac{2}{81}6^{n-1}\]

Answer 13

but 2/81 + 6 = 488/81

Answer 14

it's not in the order of 2/81, 4/27

Answer 15

ratio you multily don't add

Answer 16

its geometric!!!

Answer 17

ohhhhhhhhhhhhhhhhhh!!!!!

Answer 18

2/81 x 6 = 4/27

Answer 19

omg, thank you!!

Answer 20

I'm so sorry D;

Answer 21

thats ok