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Given the sequence: [tex]125, 25, 5, \ldots[/tex]

Which of the following rules [tex]j(n)[/tex] uses the geometric sequence formula?

A. [tex]j(n) = 125 \cdot \left(\frac{1}{5}\right)^{n-1}[/tex]

B. [tex]j(n) = 625 \cdot \left(\frac{1}{5}\right)^{n-1}[/tex]

C. [tex]j(n) = 625 \cdot 5^{n-1}[/tex]

D. [tex]j(n) = 625 \cdot \left(\frac{1}{5}\right)^n[/tex]

E. [tex]j(n) = 125 + 5n[/tex]

Answer :

To determine the rule for the given sequence [tex]\(125, 25, 5, \ldots\)[/tex], we need to verify if it follows a geometric progression and find the formula that represents it.

### Step-by-Step Solution:

1. Identify the terms:

You have a sequence starting with the numbers 125, 25, and 5.

2. Calculate the common ratio:

In a geometric sequence, each term after the first is obtained by multiplying the previous term by a constant called the common ratio ([tex]\(r\)[/tex]).

- Calculate the ratio between the second term and the first term:

[tex]\[
r = \frac{25}{125} = \frac{1}{5}
\][/tex]

- Calculate the ratio between the third term and the second term to confirm it is consistent:

[tex]\[
\frac{5}{25} = \frac{1}{5}
\][/tex]

Since both calculations give the same result, [tex]\(\frac{1}{5}\)[/tex], we confirm the sequence is geometric with a common ratio of [tex]\(\frac{1}{5}\)[/tex].

3. Write the general formula:

The formula for the [tex]\(n\)[/tex]-th term of a geometric sequence is:

[tex]\[
j(n) = a \cdot r^{(n-1)}
\][/tex]

where [tex]\(a\)[/tex] is the first term of the sequence.

- Here, [tex]\(a = 125\)[/tex] and [tex]\(r = \frac{1}{5}\)[/tex].

So, the formula becomes:

[tex]\[
j(n) = 125 \cdot \left(\frac{1}{5}\right)^{n-1}
\][/tex]

4. Choose the correct option:

After determining the formula, we identify it with option A:

[tex]\[
j(n) = 125 \cdot \left(\frac{1}{5}\right)^{n-1}
\][/tex]

Thus, the rule for the sequence is given by option A.

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