What Is The Common Ratio Of The Sequence 6 54

In mathematics, a sequence is an ordered list of numbers, called elements or terms. A common ratio is a constant value that each term in a geometric sequence is multiplied by to get the next term. Understanding the common ratio is fundamental to grasping geometric sequences and series.

Understanding Sequences and Ratios

A sequence is essentially a list of numbers that follow a certain pattern. Sequences can be finite (having a limited number of terms) or infinite (going on forever). There are several types of sequences, including arithmetic sequences, geometric sequences, harmonic sequences, and Fibonacci sequences.

A ratio, on the other hand, is a comparison of two quantities. It shows how much of one thing there is compared to another. Ratios can be expressed in different ways, such as fractions, decimals, or using a colon (:).

The Common Ratio in Geometric Sequences

A geometric sequence is a sequence where each term is obtained by multiplying the previous term by a constant value. This constant value is known as the common ratio, often denoted as r. The common ratio is crucial for understanding and working with geometric sequences.

The formula to find the nth term ((a_n)) of a geometric sequence is:

[ a_n = a_1 \cdot r^{(n-1)} ]

Where:

• (a_n) is the nth term of the sequence.
• (a_1) is the first term of the sequence.
• r is the common ratio.
• n is the term number.

Determining the Common Ratio for the Sequence 6, 54, ...

To determine the common ratio of the sequence 6, 54, ..., we need to understand how each term relates to the previous one. In a geometric sequence, you can find the common ratio by dividing any term by its preceding term.

Step-by-Step Calculation

Given the sequence 6, 54, ...:

1. Identify the terms:

   • The first term ((a_1)) is 6.
   • The second term ((a_2)) is 54.

2. Calculate the common ratio (r):

   • The common ratio r can be found by dividing the second term by the first term:

   [ r = \frac{a_2}{a_1} = \frac{54}{6} = 9 ]

Therefore, the common ratio of the sequence 6, 54, ... is 9. This means each term is multiplied by 9 to get the next term.

Verifying the Common Ratio

To verify that 9 is indeed the common ratio, we can multiply the first term (6) by 9 to see if we get the second term (54):

[ 6 \times 9 = 54 ]

Since this holds true, the common ratio r = 9 is correct.

Extending the Sequence

With the common ratio identified, we can extend the sequence by continuing to multiply each term by 9:

• Third term ((a_3)): (54 \times 9 = 486)
• Fourth term ((a_4)): (486 \times 9 = 4374)
• Fifth term ((a_5)): (4374 \times 9 = 39366)

Thus, the sequence continues as 6, 54, 486, 4374, 39366, and so on.

Importance of the Common Ratio

The common ratio is essential for several reasons:

• Predicting future terms: Once you know the common ratio, you can predict any term in the sequence without having to list all the preceding terms.
• Understanding sequence behavior: The common ratio determines whether the sequence is increasing (if r > 1), decreasing (if 0 < r < 1), or oscillating (if r < 0).
• Calculating sums of geometric series: The common ratio is used in the formula to find the sum of a finite or infinite geometric series.

Applications in Real Life

Geometric sequences and common ratios have numerous applications in real life:

• Finance: Compound interest calculations involve geometric sequences. The future value of an investment grows geometrically with a common ratio based on the interest rate.
• Population Growth: Population growth can sometimes be modeled using geometric sequences, where the common ratio represents the growth rate.
• Physics: Radioactive decay follows a geometric progression, with the common ratio being the fraction of the substance that remains after each half-life.
• Computer Science: In algorithms and data structures, geometric sequences can be used to analyze the performance and efficiency of certain processes.

Advanced Concepts Related to Geometric Sequences

Geometric Series

A geometric series is the sum of the terms in a geometric sequence. The sum of the first n terms of a geometric series ((S_n)) can be calculated using the formula:

[ S_n = \frac{a_1(1 - r^n)}{1 - r} ]

Where:

• (S_n) is the sum of the first n terms.
• (a_1) is the first term.
• r is the common ratio.
• n is the number of terms.

For example, to find the sum of the first 5 terms of the sequence 6, 54, 486, 4374, 39366:

[ S_5 = \frac{6(1 - 9^5)}{1 - 9} = \frac{6(1 - 59049)}{-8} = \frac{6(-59048)}{-8} = 44286 ]

Thus, the sum of the first 5 terms is 44286.

Infinite Geometric Series

An infinite geometric series is a geometric series with an infinite number of terms. The sum of an infinite geometric series can be found if the absolute value of the common ratio is less than 1 ((|r| < 1)). The formula for the sum of an infinite geometric series ((S_\infty)) is:

[ S_\infty = \frac{a_1}{1 - r} ]

If (|r| \geq 1), the infinite geometric series does not have a finite sum.

Convergence and Divergence

• A convergent sequence is a sequence that approaches a finite limit as the number of terms increases. In the context of geometric sequences, an infinite geometric series converges if (|r| < 1).
• A divergent sequence is a sequence that does not approach a finite limit. An infinite geometric series diverges if (|r| \geq 1).

Practical Examples and Exercises

To reinforce the understanding of common ratios, let's explore a few practical examples and exercises.

Example 1: Finding the Common Ratio

Given the sequence 3, 12, 48, ..., find the common ratio.

1. Identify the terms:

   • The first term ((a_1)) is 3.
   • The second term ((a_2)) is 12.

2. Calculate the common ratio (r):

[ r = \frac{a_2}{a_1} = \frac{12}{3} = 4 ]

The common ratio is 4.

Example 2: Extending a Sequence

Given the sequence 2, -6, 18, ..., find the next three terms.

1. Identify the terms:

   • The first term ((a_1)) is 2.
   • The second term ((a_2)) is -6.

2. Calculate the common ratio (r):

[ r = \frac{a_2}{a_1} = \frac{-6}{2} = -3 ]

3. Extend the sequence:
   • Fourth term ((a_4)): (18 \times -3 = -54)
   • Fifth term ((a_5)): (-54 \times -3 = 162)
   • Sixth term ((a_6)): (162 \times -3 = -486)

The next three terms are -54, 162, and -486.

Exercise 1: Finding the Common Ratio

Find the common ratio of the sequence 5, 15, 45, ...

Exercise 2: Extending the Sequence

Given the sequence 1, -4, 16, ..., find the next three terms.

Exercise 3: Application in Finance

An investment of $100 grows at a rate of 5% per year, compounded annually. What is the common ratio for the sequence of yearly investment values, and what will be the value of the investment after 5 years?

Common Mistakes to Avoid

When working with geometric sequences and common ratios, several common mistakes can occur. Here are a few to watch out for:

• Incorrectly calculating the common ratio: Make sure to divide a term by its preceding term, not any other term.
• Confusing geometric and arithmetic sequences: Remember that geometric sequences involve multiplication by a common ratio, while arithmetic sequences involve addition of a common difference.
• Forgetting the sign of the common ratio: The common ratio can be negative, which affects the sequence's pattern.
• Misapplying the geometric series formula: Ensure you use the correct formula for the sum of a geometric series, especially when dealing with infinite series.
• Assuming all sequences are geometric: Not all sequences are geometric; some may be arithmetic, harmonic, or follow a different pattern altogether.

The Mathematical Underpinning

The concept of the common ratio in geometric sequences is deeply rooted in mathematical principles. Geometric sequences are a specific type of exponential function, where the common ratio is the base of the exponential term. Understanding exponential growth and decay is crucial in various fields, including finance, biology, and physics.

Formal Definition

Formally, a geometric sequence ({a_n}) is defined as:

[ a_n = a_1 \cdot r^{(n-1)} ]

Where (a_1) is the initial term, r is the common ratio, and n is the term number.

Properties of Geometric Sequences

• Monotonicity: If r > 1, the sequence is monotonically increasing. If 0 < r < 1, the sequence is monotonically decreasing. If r < 0, the sequence is alternating.
• Boundedness: A geometric sequence is bounded if and only if (|r| \leq 1).
• Convergence: An infinite geometric sequence converges to 0 if (|r| < 1).

Proofs and Derivations

The formula for the sum of a geometric series can be derived as follows:

Let (S_n) be the sum of the first n terms of a geometric series:

[ S_n = a_1 + a_1r + a_1r^2 + \cdots + a_1r^{(n-1)} ]

Multiply both sides by r:

[ rS_n = a_1r + a_1r^2 + a_1r^3 + \cdots + a_1r^n ]

Subtract the second equation from the first:

[ S_n - rS_n = a_1 - a_1r^n ]

Factor out (S_n) on the left and (a_1) on the right:

[ S_n(1 - r) = a_1(1 - r^n) ]

Divide by ((1 - r)):

[ S_n = \frac{a_1(1 - r^n)}{1 - r} ]

This is the formula for the sum of the first n terms of a geometric series.

Conclusion

The common ratio is a fundamental concept in geometric sequences, playing a crucial role in understanding patterns, predicting future terms, and calculating sums of series. In the sequence 6, 54, ..., the common ratio is 9. This knowledge enables us to extend the sequence, understand its behavior, and apply it to various real-world problems. By mastering the common ratio, one can unlock deeper insights into mathematical sequences and their applications in diverse fields.