Arithmetic & Geometric Sequences - Algebra 2

Section 1

Historical Context & Motivation

Sequences — ordered lists of numbers following a definite rule — are among the oldest mathematical objects ever studied. Long before algebra was formalized, ancient civilizations recognized that patterns in nature and commerce could be captured by sequences. The two most fundamental types, arithmetic sequences (constant addition) and geometric sequences (constant multiplication), appear in everything from architecture to finance. Understanding them historically illuminates why mathematicians developed two different ways to describe them: recursive formulas (describing each term from the previous one) and explicit formulas (jumping directly to any term).

c. 1650 BCE

The Rhind Mathematical Papyrus of ancient Egypt contains problems involving arithmetic progressions — for example, dividing loaves of bread among workers using an evenly increasing distribution. This is among the earliest recorded uses of arithmetic sequences in a practical context.

c. 300 BCE

Euclid, in Elements (Books VII–IX), studied "continued proportions" — what we now call geometric sequences. He proved foundational results about ratios and established that the product of the extremes equals the product of the means in a geometric proportion.

c. 1780

A young Carl Friedrich Gauss, according to legend, astonished his teacher by quickly summing the integers from 1 to 100. His insight — pairing the first and last terms to form equal sums — formalized the arithmetic series sum formula and demonstrated the power of the explicit approach to sequences.

1202 CE

Leonardo of Pisa (Fibonacci) published Liber Abaci, introducing what is now called the Fibonacci sequence — a famous recursively defined sequence. While not arithmetic or geometric, it demonstrated the power and elegance of recursive thinking to European mathematics.

Modern Era

Today, arithmetic and geometric sequences underpin compound interest calculations, population models, computer science algorithms, and signal processing. The recursive–explicit duality reappears throughout higher mathematics, from difference equations to generating functions.

The central challenge that arithmetic and geometric sequences address is this: given a pattern that grows (or shrinks) in a predictable way, how can we describe the pattern completely, find any term we want, and sum up as many terms as we need? Two complementary tools — recursive formulas and explicit formulas — give us complete mastery over these patterns.

Section 2

Core Principles & Definitions

Before diving into formulas, we need rock-solid definitions. A sequence is an ordered list of numbers called terms, typically denoted a₁, a₂, a₃, … , aₙ. The subscript n is the term number (or index), and aₙ represents the nth term. Two fundamental questions arise for any sequence: "How does each term relate to the one before it?" and "Can I jump directly to the 100th term without computing the first 99?" These questions lead to our two formula types.

Arithmetic Sequence

A sequence where each term is obtained by adding a fixed number (the common difference, d) to the preceding term. Example: 3, 7, 11, 15, … where d = 4.

Geometric Sequence

A sequence where each term is obtained by multiplying the previous term by a fixed number (the common ratio, r). Example: 2, 6, 18, 54, … where r = 3.

Recursive Formula

Defines each term in relation to the previous term(s). Requires a starting value (initial condition). Like directions that say "from where you are now, take one more step forward."

Explicit Formula

Gives the nth term directly as a function of n — no need to know any other terms. Like a GPS that teleports you straight to your destination.

✦

✦ Key Takeaway

Think of a recursive formula as step-by-step walking directions ("turn left, walk 3 blocks, turn right…") and an explicit formula as a GPS coordinate. Both describe the same destination, but the GPS lets you jump there instantly. Arithmetic sequences add a constant; geometric sequences multiply by a constant. That's the single most important distinction.

Section 3

Visual Explanation

The diagram below visualizes how arithmetic and geometric sequences grow. On the left, an arithmetic sequence increases by a constant step (the common difference d), producing a straight line when plotted. On the right, a geometric sequence multiplies by a constant ratio (the common ratio r), producing an exponential curve. This visual difference is fundamental: arithmetic growth is linear, while geometric growth is exponential.

Comparison graph showing arithmetic sequence as a straight line and geometric sequence as an exponential curve.

Notice how the arithmetic sequence forms a perfectly straight line — each point is the same vertical distance from the last. The geometric sequence, on the other hand, curves upward dramatically. By term 6, the geometric sequence has already reached 32 while the arithmetic sequence is only at 17. This visual difference captures a profound truth: additive growth is steady; multiplicative growth is explosive. This is why understanding both types — and their formulas — is essential in fields from finance (compound vs. simple interest) to biology (population growth vs. linear approximations).

Section 4

Mathematical Framework — The Four Formulas

Each sequence type has two formula forms: recursive and explicit. Together, these four formulas form the complete toolkit for arithmetic and geometric sequences. Let us develop each one from first principles, starting with the recursive definitions and then deriving the explicit forms.

Arithmetic Sequence Formulas

In an arithmetic sequence, every term equals the previous term plus the common difference d. If you know a₁ (the first term) and d, you can construct the entire sequence one step at a time.

Arithmetic — Recursive Formula

aₙ = aₙ₋₁ + d, where a₁ is given

Each term = previous term + common difference. Requires the initial term a₁.

While the recursive formula is intuitive — it tells you exactly how to build the next term — it has a practical drawback: to find the 100th term, you'd have to compute all 99 terms before it. The explicit formula solves this by observing that term n is simply the first term plus (n − 1) copies of d added on.

Arithmetic — Explicit Formula

aₙ = a₁ + (n − 1) × d

a₁ = first term, d = common difference, n = term number. Gives any term directly.

The logic is straightforward: to get from a₁ to aₙ, you must add d exactly (n − 1) times. For example, to reach a₅, you add d four times to a₁: a₅ = a₁ + 4d.

Geometric Sequence Formulas

In a geometric sequence, every term equals the previous term multiplied by the common ratio r. The recursive definition is the natural starting point.

Geometric — Recursive Formula

aₙ = aₙ₋₁ × r, where a₁ is given

Each term = previous term × common ratio. Requires the initial term a₁.

Again, the recursive form is wonderful for understanding the pattern but cumbersome for finding distant terms. The explicit formula arises from recognizing that to reach term n, you multiply a₁ by r exactly (n − 1) times.

Geometric — Explicit Formula

aₙ = a₁ × r^(n−1)

a₁ = first term, r = common ratio, n = term number. Gives any term directly.

Notice the elegant parallel between the two explicit formulas. In the arithmetic version, we add d a total of (n − 1) times; in the geometric version, we multiply by r a total of (n − 1) times. This is the essence of the distinction: arithmetic sequences are built on repeated addition, while geometric sequences are built on repeated multiplication.

✦

✦ Key Takeaway

The recursive formula tells you the rule for building the sequence one step at a time — like knowing "add 3 each time" or "multiply by 2 each time." The explicit formula is the shortcut that lets you calculate the value of any term directly, without computing every previous term. Both are equivalent descriptions of the same sequence; choosing which to use depends on whether you need the next term or a distant one.

Section 5

Detailed Breakdown & Classification

Let us explore the anatomy of each formula type more deeply and see how to identify, classify, and convert between them. The diagram below presents a decision flowchart: given a sequence, how do you determine its type and write both formulas?

Flowchart showing how to classify a sequence as arithmetic or geometric and derive recursive and explicit formulas.

The key diagnostic step is simple. Take consecutive terms and compute their differences (a₂ − a₁, a₃ − a₂, etc.) and their ratios (a₂ / a₁, a₃ / a₂, etc.). If the differences are all the same, the sequence is arithmetic with that common difference. If the ratios are all the same, the sequence is geometric with that common ratio. If neither, the sequence is neither type.

Identifying d and r — Worked Through

Consider the sequence 5, 11, 17, 23, 29, … Computing differences: 11 − 5 = 6, 17 − 11 = 6, 23 − 17 = 6, 29 − 23 = 6. Every difference is 6, so this is an arithmetic sequence with a₁ = 5 and d = 6. Recursive: aₙ = aₙ₋₁ + 6, a₁ = 5. Explicit: aₙ = 5 + (n − 1) × 6 = 6n − 1.

Now consider 3, 12, 48, 192, 768, … Computing ratios: 12/3 = 4, 48/12 = 4, 192/48 = 4, 768/192 = 4. Every ratio is 4, so this is a geometric sequence with a₁ = 3 and r = 4. Recursive: aₙ = 4 × aₙ₋₁, a₁ = 3. Explicit: aₙ = 3 × 4⁽ⁿ⁻¹⁾.

Special Cases to Watch

A constant sequence like 7, 7, 7, 7, … is both arithmetic (d = 0) and geometric (r = 1). A geometric sequence with r = −1 alternates signs: 5, −5, 5, −5, …. A geometric sequence with |r| < 1 converges toward zero: 100, 50, 25, 12.5, …. Meanwhile, a negative common difference in an arithmetic sequence produces a decreasing sequence: 40, 33, 26, 19, ….

Section 6

Worked Example

Let's work through a comprehensive problem that exercises all four formulas. This problem demonstrates how to extract both recursive and explicit formulas for each type, and how to use the explicit formula to find a distant term efficiently.

Problem: A theater has 20 seats in the first row, 24 in the second, 28 in the third, and so on. A second theater doubles its seating: 5 seats in row 1, 10 in row 2, 20 in row 3, and so on. For each theater, write recursive and explicit formulas, then find the number of seats in row 15.

1
Step 1 — Classify Theater A

The sequence is 20, 24, 28, … . The differences are 24 − 20 = 4 and 28 − 24 = 4, so this is an arithmetic sequence with a₁ = 20 and d = 4.
2
Step 2 — Write Theater A's Formulas

Recursive: aₙ = aₙ₋₁ + 4, a₁ = 20 Explicit: aₙ = 20 + (n − 1) × 4 = 4n + 16
3
Step 3 — Find Row 15 for Theater A

Using the explicit formula:

a₁₅ = 4(15) + 16 = 60 + 16 = 76 seats
4
Step 4 — Classify Theater B

The sequence is 5, 10, 20, … . The ratios are 10/5 = 2 and 20/10 = 2, so this is a geometric sequence with a₁ = 5 and r = 2.
5
Step 5 — Write Theater B's Formulas

Recursive: aₙ = 2 × aₙ₋₁, a₁ = 5 Explicit: aₙ = 5 × 2^(n−1)
6
Step 6 — Find Row 15 for Theater B

Using the explicit formula: a₁₅ = 5 × 2¹⁴ = 5 × 16,384 = 81,920 seats

Of course, 81,920 seats in a single row is absurd in practice — this illustrates how quickly geometric growth outpaces arithmetic growth! By row 15, Theater A adds a modest 56 seats beyond its starting row, while Theater B's exponential growth has produced an astronomically large (and impractical) number.

Section 7

Strengths, Limitations & Comparison

Now that we understand all four formulas, let's compare them side by side. Each formula type has its own strengths and ideal use cases. The table below provides a comprehensive comparison.

Feature	Arithmetic	Geometric
Operation	Repeated addition (+d)	Repeated multiplication (×r)
Graph shape	Linear (straight line)	Exponential (curve)
Recursive form	aₙ = aₙ₋₁ + d	aₙ = r × aₙ₋₁
Explicit form	aₙ = a₁ + (n−1)d	aₙ = a₁ × r⁽ⁿ⁻¹⁾
Key parameter	Common difference (d)	Common ratio (r)
Growth rate	Constant (same amount each step)	Accelerating (proportional to current value)
Real-world example	Salary with fixed annual raise	Compound interest on investment
Can terms be negative?	Yes (if d < 0 eventually)	Yes (if r < 0, terms alternate signs)
When to use recursive	Modeling step-by-step processes, spreadsheets	Population growth models, iterative algorithms
When to use explicit	Finding a specific far-off term directly	Computing value after many periods (e.g., 30-year investment)

Recursive vs. Explicit: Pros and Cons

Aspect	Recursive Formula	Explicit Formula
Intuition	✅ Very intuitive — describes the pattern step by step	⚠️ Requires algebraic derivation
Finding term n	❌ Must compute all preceding terms	✅ Direct calculation — O(1) for arithmetic, O(log n) for geometric
Memory	⚠️ Need to track previous term(s)	✅ Only need a₁ and d (or r)
Generalizability	✅ Works for many sequence types (Fibonacci, etc.)	⚠️ Not all recursions have clean closed forms

✦

✦ Key Takeaway

Neither formula type is "better" — they're complementary tools. Use the recursive formula when you're building a sequence term by term (as in a spreadsheet or simulation) or when the problem naturally describes how each term relates to the previous one. Use the explicit formula when you need to jump to a specific term, find which term equals a given value, or analyze the long-term behavior of the sequence.

Section 8

Connection to Advanced Theory

Arithmetic and geometric sequences are the foundation upon which many advanced mathematical concepts are built. Understanding them deeply prepares you for several major topics you will encounter in precalculus, calculus, and beyond.

Series and Summation. When you add up the terms of a sequence, you get a series. The sum of the first n terms of an arithmetic sequence is Sₙ = n(a₁ + aₙ)/2 (Gauss's insight!), while the partial sum of a geometric series is Sₙ = a₁(1 − rⁿ)/(1 − r). These summation formulas are direct extensions of the sequence formulas studied here.

Infinite Geometric Series. When |r| < 1, a geometric series converges to a finite sum: S = a₁/(1 − r). This remarkable result connects sequences to the concept of limits — a cornerstone of calculus. For example, the series ½ + ¼ + ⅛ + … converges to exactly 1.

Linear and Exponential Functions. An arithmetic sequence's explicit formula aₙ = a₁ + (n − 1)d is a linear function of n. A geometric sequence's explicit formula aₙ = a₁ × r⁽ⁿ⁻¹⁾ is an exponential function of n. This parallel is why studying sequences illuminates the broader relationship between linear and exponential growth — a theme central to Algebra 2 and precalculus.

Concept	Sequences Foundation	Advanced Extension
Arithmetic Sequence	aₙ = a₁ + (n−1)d	Linear functions f(x) = mx + b
Geometric Sequence	aₙ = a₁ × r⁽ⁿ⁻¹⁾	Exponential functions f(x) = ab^x
Recursive definitions	aₙ = aₙ₋₁ + d or aₙ₋₁ × r	Difference equations, dynamical systems
Finite sums (series)	Sₙ formulas for arith./geom.	Summation notation (Σ), integral calculus
Convergence (\|r\| < 1)	Terms approach zero	Infinite series, limits, Taylor series

In more advanced courses, you'll encounter sequences that are neither arithmetic nor geometric — like the Fibonacci sequence (where aₙ = aₙ₋₁ + aₙ₋₂) — and discover that finding explicit formulas for such sequences requires techniques from linear algebra. The recursive–explicit duality you're learning now is your first encounter with one of mathematics' deepest themes: describing patterns both locally (what happens next?) and globally (what's the big picture?).

Section 9

Practice Problems

Work through these five problems in order. They progress from conceptual understanding to multi-step applications and critical thinking. Try each problem on your own before revealing the answer.

Question 1: Problem 1 — Conceptual

Explain in your own words the difference between a recursive formula and an explicit formula. Why might you prefer one over the other in a given situation?

Show answer

A recursive formula defines each term in a sequence based on the previous term (or terms). It tells you how to get the next value from the current one, and it always requires an initial condition (the value of a₁). For example, "start at 3, add 5 each time." An explicit formula expresses the nth term directly as a function of the term number n, allowing you to calculate any term without knowing any others. For example, aₙ = 3 + 5(n − 1). You'd prefer the recursive formula when simulating a process step by step (like a computer program iterating through values) or when the relationship between consecutive terms is the most natural description. You'd prefer the explicit formula when you need to find a specific term far into the sequence (like the 500th term) without computing all the preceding ones, or when solving for which term equals a given value.

Question 2: Problem 2 — Basic Calculation

The arithmetic sequence begins 7, 13, 19, 25, … . Write the recursive formula and the explicit formula, then find a₂₀.

Show answer

First, identify the common difference: d = 13 − 7 = 6. The first term is a₁ = 7. Recursive formula: aₙ = aₙ₋₁ + 6, a₁ = 7. Explicit formula: aₙ = 7 + (n − 1) × 6 = 6n + 1. Finding a₂₀: a₂₀ = 6(20) + 1 = 120 + 1 = 121. You can verify: the first few terms are 7, 13, 19, 25, 31, … and 6(1)+1 = 7 ✓, 6(2)+1 = 13 ✓. The formula checks out.

Question 3: Problem 3 — Intermediate

A geometric sequence has a₃ = 36 and a₆ = 972. Find the common ratio r, the first term a₁, and write both the recursive and explicit formulas.

Show answer

Using the explicit formula, a₆ = a₁ × r⁵ and a₃ = a₁ × r². Dividing: a₆/a₃ = r³ = 972/36 = 27, so r = 3. Now find a₁: a₃ = a₁ × 3² = 9a₁ = 36, so a₁ = 36/9 = 4. Recursive formula: aₙ = 3 × aₙ₋₁, a₁ = 4. Explicit formula: aₙ = 4 × 3⁽ⁿ⁻¹⁾. Verification: a₃ = 4 × 3² = 4 × 9 = 36 ✓. a₆ = 4 × 3⁵ = 4 × 243 = 972 ✓.

Question 4: Problem 4 — Applied / Multi-Step

A car depreciates in value each year. It was worth $32,000 new, $24,000 after one year, and $18,000 after two years. Is this depreciation arithmetic or geometric? Write the explicit formula and predict the car's value after 6 years (round to the nearest dollar).

Show answer

Testing arithmetic: 24,000 − 32,000 = −8,000 and 18,000 − 24,000 = −6,000. The differences are not equal, so it is not arithmetic. Testing geometric: 24,000/32,000 = 0.75 and 18,000/24,000 = 0.75. The ratios are equal! This is a geometric sequence with a₁ = 32,000 and r = 0.75. Explicit formula: aₙ = 32,000 × (0.75)⁽ⁿ⁻¹⁾, where n = 1 corresponds to the car when new. After 6 years: The value at year 7 (n = 7, since n = 1 is "new"): a₇ = 32,000 × (0.75)⁶ = 32,000 × 0.177979 ≈ $5,695. Notice how the geometric ratio r = 0.75 (less than 1) causes the value to decay — each year the car retains only 75% of its previous year's value. This is a classic real-world geometric sequence with |r| < 1.

Question 5: Problem 5 — Critical Thinking / Synthesis

A student claims: "If I know a₁ = 10 and a₅ = 810, the sequence must be geometric with r = 3." Is the student correct? Could the sequence be arithmetic instead? Justify your answer and, if the student is correct about it being geometric, write both formulas.

Show answer

Testing geometric: If geometric, a₅ = a₁ × r⁴, so 810 = 10 × r⁴, giving r⁴ = 81 and r = ±3. The student says r = 3, which is one valid solution. However, r = −3 is also possible (since (−3)⁴ = 81), which would produce the sequence 10, −30, 90, −270, 810 — an alternating-sign geometric sequence. Testing arithmetic: If arithmetic, a₅ = a₁ + 4d, so 810 = 10 + 4d, giving d = 200. That yields the sequence 10, 210, 410, 610, 810 — a perfectly valid arithmetic sequence. Therefore, the student is not entirely correct. Knowing only a₁ and a₅ does not uniquely determine the type. The sequence could be arithmetic (d = 200) or geometric (r = 3 or r = −3). Additional terms are needed to determine the type with certainty. If we assume it's geometric with r = 3: Recursive: aₙ = 3 × aₙ₋₁, a₁ = 10. Explicit: aₙ = 10 × 3⁽ⁿ⁻¹⁾. This critical thinking problem illustrates that identifying a sequence type requires checking the pattern across multiple consecutive terms, not just two data points.

Summary

Lesson Summary

Summary

An arithmetic sequence grows by adding a constant called the common difference (d), while a geometric sequence grows by multiplying by a constant called the common ratio (r). Each type can be expressed in two equivalent ways. The recursive formula defines each term from its predecessor — aₙ = aₙ₋₁ + d for arithmetic and aₙ = r × aₙ₋₁ for geometric — and requires a starting value a₁. The explicit formula gives any term directly: aₙ = a₁ + (n − 1)d for arithmetic and aₙ = a₁ × r⁽ⁿ⁻¹⁾ for geometric.

To classify a sequence, check whether consecutive differences are constant (arithmetic) or consecutive ratios are constant (geometric). Visually, arithmetic sequences produce straight lines and geometric sequences produce exponential curves. Recursive formulas excel at modeling step-by-step processes, while explicit formulas excel at direct computation of distant terms. Together, these four formulas — the twin descriptions of the two most fundamental sequence types — form the gateway to series, summation, limits, and the broader world of functions and calculus.