Question

What is the explicit rule for this geometric sequence?

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Answer 1

Why not use n = 1 in the formula and see if you get \{a_{1} = -15\}?

Answer 2

@3mar

Answer 3

Well, I am here.

What do you think?
Any ideas?

Answer 4

C?

Answer 5

Correct!
You hit the right target!
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Answer 6

Try working through this recursively:
\[\begin{align*}
a_n&=\frac{1}{5}a_{n-1}\\[1ex]
&=\frac{1}{5}\left(\frac{1}{5}a_{n-2}\right)\\[1ex]
&=\frac{1}{5^2}a_{n-2}\\[1ex]
&=\frac{1}{5^2}\left(\frac{1}{5}a_{n-3}\right)\\[1ex]
&=\frac{1}{5^3}a_{n-3}
\end{align*}\]Notice a pattern yet?