Question

anyone help me
Write the explicit formula for the geometric sequence. a1 = -5 a2 = 20 a3 = -80

Answer 1

what's the formula for geometric sequence ??

Answer 2

For r\neq 1, the sum of the first n terms of a geometric series is:

a + ar + a r^2 + a r^3 + \cdots + a r^{n-1} = \sum_{k=0}^{n-1} ar^k= a \, \frac{1-r^{n}}{1-r},

Answer 3

\[a_n = a_1 \times (r)^{n-1}\] formula
where a_1 is first term
and r is ratio

Answer 4

what's the ratio for that sequence ?
to find ratio \[\huge\rm r = \frac{ a_2 }{ a_1 }\]
divide next term by previous one
2nd term/1st term

Answer 5

If it's asking for an explicit formula, it probably wants you to identify \(a\) and \(r\) then use summation notation instead of writing the series as a sum of individual terms: \(\sum_{n=1}^{3}a_n\) where \(a_n=a\cdot r^n\).

Answer 6

okay

Answer 7

i guess they just need formula a_1 is already there

Answer 8

thax for the help

Answer 9

what's "r" equal to ??
r= ??

Answer 10

i will find out i know all this just that i was a liitle cunfused

Answer 11

okay...
let me know what u get :-)
take ur time

Answer 12

here are the answers
A an = -5* (-4)^n
B an = -5(-4)^n-1
C an = -4(-5)^n-1
i got D wrong

Answer 13

that isn't answer of my questions :(

Answer 14

okay so r = ???

Answer 15

Ok, back.
So we have alternating positive and negative numbers, so you know you have a \((-1)^n\) in the explicit formula. Now 5, 20, 80... all have a factor of 5 in common:

5(1) =5
5(4) =20
5(16) = 80

Does this help?