Finding the Explicit Formula

Oct 13, 2025 by wbrum.org 9 views

Introduction

In mathematics, a geometric sequence is a type of sequence where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the common ratio. The formula $f(x+1)=\frac{2}{3}(f(x)$ defines a geometric sequence, and we are given that $f(1)=18$ . In this article, we will explore how to find the explicit formula that can be used to model the same sequence.

Understanding the Formula

The given formula $f(x+1)=\frac{2}{3}(f(x)$ tells us that to find the next term in the sequence, we need to multiply the current term by $\frac{2}{3}$ . This means that the common ratio of the sequence is $\frac{2}{3}$ .

Finding the Explicit Formula

To find the explicit formula for the sequence, we can use the formula for a geometric sequence, which is given by:

f(x) = f(1) \cdot r^{x-1}

where $f(1)$ is the first term of the sequence, $r$ is the common ratio, and $x$ is the term number.

In this case, we are given that $f(1)=18$ and $r=\frac{2}{3}$ . Plugging these values into the formula, we get:

f(x) = 18 \cdot \left(\frac{2}{3}\right)^{x-1}

Simplifying the Formula

We can simplify the formula by rewriting the common ratio as a power of 2:

\left(\frac{2}{3}\right)^{x-1} = \left(\frac{2}{3}\right)^{x-1} \cdot \frac{3}{3} = \frac{2^{x-1}}{3^{x-1}} \cdot \frac{3}{3} = \frac{2^{x-1}}{3^{x-2}}

Substituting this back into the formula, we get:

f(x) = 18 \cdot \frac{2^{x-1}}{3^{x-2}}

Conclusion

In this article, we have found the explicit formula for the geometric sequence defined by the formula $f(x+1)=\frac{2}{3}(f(x)$ . The formula is given by:

f(x) = 18 \cdot \frac{2^{x-1}}{3^{x-2}}

This formula can be used to model the same sequence, and it provides a clear and concise way to calculate any term in the sequence.

The Final Answer

The final answer is:

f(x) = 18 \cdot \frac{2^{x-1}}{3^{x-2}}

References

[1] "Geometric Sequences" by Math Open Reference
[2] "Geometric Sequences" by Khan Academy

Discussion

Do you have any questions about geometric sequences or the formula we derived?
Can you think of any other ways to simplify the formula?
How do you think the formula can be used in real-world applications?

The Formula for an Arithmetic Sequence
The Formula for a Geometric Sequence
The Formula for a Harmonic Sequence
Q&A: Geometric Sequences and the Formula =============================================

Introduction

In our previous article, we explored the formula for a geometric sequence and derived the explicit formula for the sequence defined by $f(x+1)=\frac{2}{3}(f(x)$ . In this article, we will answer some common questions about geometric sequences and the formula.

Q: What is a geometric sequence?

A: A geometric sequence is a type of sequence where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the common ratio.

Q: What is the formula for a geometric sequence?

A: The formula for a geometric sequence is given by:

f(x) = f(1) \cdot r^{x-1}

where $f(1)$ is the first term of the sequence, $r$ is the common ratio, and $x$ is the term number.

Q: How do I find the common ratio of a geometric sequence?

A: To find the common ratio of a geometric sequence, you can use the formula:

r = \frac{f(x+1)}{f(x)}

where $f(x+1)$ is the next term in the sequence and $f(x)$ is the current term.

Q: How do I find the explicit formula for a geometric sequence?

A: To find the explicit formula for a geometric sequence, you can use the formula:

f(x) = f(1) \cdot r^{x-1}

where $f(1)$ is the first term of the sequence and $r$ is the common ratio.

Q: What is the difference between an arithmetic sequence and a geometric sequence?

A: An arithmetic sequence is a type of sequence where each term after the first is found by adding a fixed number to the previous term. A geometric sequence is a type of sequence where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the common ratio.

Q: Can you give an example of a geometric sequence?

A: Yes, here is an example of a geometric sequence:

1, 2, 4, 8, 16, ...

This sequence is a geometric sequence with a common ratio of 2.

Q: How do I use the formula to find the next term in a geometric sequence?

A: To use the formula to find the next term in a geometric sequence, you can plug in the values of $f(x)$ and $r$ into the formula:

f(x+1) = f(x) \cdot r

where $f(x)$ is the current term and $r$ is the common ratio.

Q: Can you give an example of how to use the formula to find the next term in a geometric sequence?

A: Yes, here is an example of how to use the formula to find the next term in a geometric sequence:

Suppose we have a geometric sequence with a common ratio of 2 and the first term is 3. To find the next term, we can plug in the values into the formula:

f(x+1) = f(x) \cdot r

f(2) = 3 \cdot 2

f(2) = 6

Therefore, the next term in the sequence is 6.

Conclusion

In this article, we have answered some common questions about geometric sequences and the formula. We hope that this article has been helpful in understanding geometric sequences and how to use the formula to find the next term in a geometric sequence.

References

[1] "Geometric Sequences" by Math Open Reference
[2] "Geometric Sequences" by Khan Academy

Discussion

Do you have any questions about geometric sequences or the formula?
Can you think of any other ways to simplify the formula?
How do you think the formula can be used in real-world applications?