what is a geometric sequence

2 min read 15-03-2025

A geometric sequence is a specific type of sequence where each term is found by multiplying the previous term by a constant value. This constant value is called the common ratio. Understanding geometric sequences is fundamental in various fields, from finance (compound interest) to computer science (recursive algorithms).

Understanding the Common Ratio

The common ratio (often denoted as 'r') is the heart of a geometric sequence. It's the number you multiply each term by to get the next. Let's look at an example:

Example: The sequence 2, 6, 18, 54... is a geometric sequence.

The first term (a₁) is 2.
To get the second term, we multiply the first term by 3: 2 * 3 = 6.
To get the third term, we multiply the second term by 3: 6 * 3 = 18.
And so on...

In this example, the common ratio (r) is 3.

Identifying Geometric Sequences

To determine if a sequence is geometric, check if there's a consistent common ratio between consecutive terms. If you divide any term by the preceding term and get the same result each time, you've got a geometric sequence.

Example: Is the sequence 1, 4, 16, 64... a geometric sequence?

4 / 1 = 4
16 / 4 = 4
64 / 16 = 4

Yes! The common ratio is 4.

The Formula for the nth Term

We can find any term in a geometric sequence using a formula:

aₙ = a₁ * rⁿ⁻¹

Where:

aₙ is the nth term in the sequence.
a₁ is the first term.
r is the common ratio.
n is the term number.

Let's use the previous example (1, 4, 16, 64...) to find the 6th term:

a₁ = 1
r = 4
n = 6

a₆ = 1 * 4⁶⁻¹ = 1 * 4⁵ = 1024

The 6th term is 1024.

Geometric Series: Summing the Terms

A geometric series is the sum of the terms in a geometric sequence. There's a handy formula for calculating the sum of the first 'n' terms of a geometric series:

Sₙ = a₁ * (1 - rⁿ) / (1 - r)

This formula works only if the absolute value of 'r' is less than 1 (|r| < 1). If |r| ≥ 1, the series does not converge to a finite sum.

Infinite Geometric Series

When the absolute value of the common ratio is less than 1 (|r| < 1), the infinite geometric series converges to a finite sum. The formula for this is:

S = a₁ / (1 - r)

This means that even though the sequence has infinitely many terms, their sum approaches a specific value.

Real-World Applications

Geometric sequences and series appear in various real-world situations:

Compound Interest: The growth of money invested with compound interest follows a geometric sequence.
Population Growth: Under certain conditions, population growth can be modeled using geometric sequences.
Radioactive Decay: The decay of radioactive materials follows a geometric sequence.
Fractals: Many fractal patterns are based on geometric sequences.

Key Differences from Arithmetic Sequences

It's important to distinguish geometric sequences from arithmetic sequences. In an arithmetic sequence, each term is found by adding a constant value (the common difference) to the previous term, not multiplying.

Understanding the distinction between these two types of sequences is crucial for accurately modeling various phenomena.

This comprehensive guide provides a strong foundation for understanding geometric sequences and their practical applications. Remember to practice with different examples to solidify your understanding.