College

A sequence is defined by the recursive function [tex]$f(n+1)=\frac{1}{3} f(n)$[/tex]. If [tex]$f(3)=9$[/tex], what is [tex][tex]$f(1)$[/tex][/tex]?

A. 81
B. 27
C. 1
D. 3

Answer :

To solve the problem, we need to determine the value of [tex]\( f(1) \)[/tex] using the recursive function defined as [tex]\( f(n+1) = \frac{1}{3} f(n) \)[/tex] and the information that [tex]\( f(3) = 9 \)[/tex].

Since we are given [tex]\( f(3) = 9 \)[/tex] and need to find [tex]\( f(1) \)[/tex], we will work backwards using the recursive relationship. The recursive formula tells us how to go from [tex]\( f(n) \)[/tex] to [tex]\( f(n+1) \)[/tex]. To find the previous terms, we need to reverse this relation.

The recursive formula [tex]\( f(n+1) = \frac{1}{3} f(n) \)[/tex] can be rearranged to find [tex]\( f(n) \)[/tex] when [tex]\( f(n+1) \)[/tex] is known:

[tex]\[ f(n) = 3 \times f(n+1) \][/tex]

Let's use this to calculate the values step by step:

1. Start with the known value: [tex]\( f(3) = 9 \)[/tex].
2. Find [tex]\( f(2) \)[/tex] using [tex]\( f(n) = 3 \times f(n+1) \)[/tex]:
[tex]\[
f(2) = 3 \times f(3) = 3 \times 9 = 27
\][/tex]
3. Next, find [tex]\( f(1) \)[/tex]:
[tex]\[
f(1) = 3 \times f(2) = 3 \times 27 = 81
\][/tex]

Therefore, the value of [tex]\( f(1) \)[/tex] is [tex]\(\boxed{81}\)[/tex].