Question

Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80
A. an = -5 • (-4)^n

B. an = -5(-4)^n-1

C. an = -4(-5)^n-1

D. an = -5 • (4)^n

Answer 1

http://openstudy.com/study#/updates/503bb2a0e4b007f9003103b0
as u can see there,the general formula is
\[T_n=a_1r^{(n-1)}\]
where a1 is 1st term=-5
r is common ratio
can u find r?

Answer 2

still lost =(

Answer 3

whats 20/(-5)
whats (-80)/20
?
common ratio r is the ratio of next term to current term.......

Answer 4

-4

Answer 5

how is that going to help ??

Answer 6

so it is not (D)

Answer 7

oh,sorry
now r=-4,a1=-5
put it in
\[T_n=a_1r^{(n-1)}\]
what u get?

Answer 8

you just lost me with that

Answer 9

or u had doubts with r=-4 ??