---
title: "Sum of Cubes | College Algebra"
description: "Sum of Cubes is the pattern a^3 + b^3 and its factoring form in College Algebra, helping you simplify polynomials and solve equations faster."
canonical: "https://fiveable.me/college-algebra/key-terms/sum-cubes"
type: "key-term"
subject: "College Algebra"
unit: "Unit 5"
---

# Sum of Cubes | College Algebra

## Definition

The sum of cubes is an expression like a^3 + b^3. In College Algebra, you usually rewrite it as (a + b)(a^2 - ab + b^2) to factor polynomials and solve equations.

## What It Is

The sum of cubes in College Algebra is a polynomial expression made by adding two cubic terms, usually written as a^3 + b^3. The main reason it shows up in this course is that you can factor it using a special pattern instead of expanding it by hand every time.

The factoring rule is a^3 + b^3 = (a + b)(a^2 - ab + b^2). The first factor keeps the same signs as the original sum, so a positive cube sum becomes a plus inside the first parentheses. The second factor has three terms, and the middle term is negative, which trips people up because the original expression has only plus signs.

A quick example is x^3 + 8. Since 8 = 2^3, you can rewrite it as x^3 + 2^3, then factor it as (x + 2)(x^2 - 2x + 4). That check is useful because multiplying the factors back together should return the original polynomial.

This pattern works only when you have a sum of two perfect cubes, not just any two terms with exponents. For example, x^3 + 27 factors because 27 is 3^3, but x^3 + 16 does not fit the sum of cubes form because 16 is not a perfect cube. If the expression does not match the pattern, you need a different factoring strategy.

You will also see the sum of cubes as part of the bigger family of special factoring formulas in polynomial work. It connects to recognizing powers, rewriting expressions in simpler forms, and deciding which factoring method fits a problem before you start expanding or solving.

## Why It Matters

The sum of cubes matters because College Algebra leans hard on pattern recognition. Once you spot a cubic sum, you can factor it quickly instead of trying trial-and-error methods that waste time and often miss the right structure.

It also shows up inside equation solving. If a problem gives you x^3 + 27 = 0, factoring turns it into (x + 3)(x^2 - 3x + 9) = 0, which lets you use the zero product property. That is much cleaner than treating the equation like a random polynomial.

This pattern builds your skill with polynomials overall. Special factoring formulas like the sum of cubes train you to look at the shape of an expression, identify perfect powers, and choose the right method. That same habit shows up again with other factoring patterns, rational expressions, and graphing work where the algebraic form matters.

It also helps you avoid common mistakes. A lot of students remember that a sum of cubes factors, but forget the exact middle sign or the squared terms. If you can write the pattern correctly, you save yourself from algebra errors later in the course.

## Connections

### Difference of Cubes

Difference of cubes is the close partner to sum of cubes, but the signs change. You use a similar factoring pattern, so it is easy to mix them up if you are only memorizing one formula. Comparing the two helps you notice that the first factor keeps the sign from the original expression, while the second factor always has the mixed middle term.

### Factoring Polynomials

Sum of cubes is one specific tool inside factoring polynomials. When a polynomial does not have a common factor, you may look for special patterns like this one. It fits into the bigger decision-making process of choosing whether to factor by grouping, use a special formula, or try another method.

### [Factoring out the GCF](/college-algebra/key-terms/factoring-gcf)

You should always check for a greatest common factor before looking for a sum of cubes. If both terms share a factor, pull that out first, because it may reveal a hidden cube pattern afterward. Skipping the GCF step can make a polynomial look more complicated than it really is.

### [Factoring Strategies](/college-algebra/key-terms/factoring-strategies)

Sum of cubes is part of the larger set of factoring strategies you use to match an expression with the right method. In practice, you scan for a GCF, special products, and grouping patterns before you start solving. This term is one of the pattern-recognition tools that makes those choices faster.

## On the AP Exam

A problem set or quiz question will usually give you a polynomial and ask you to factor it completely or solve an equation that contains a cubic sum. Your job is to recognize when both terms are perfect cubes, rewrite them in cube form if needed, and apply (a + b)(a^2 - ab + b^2) correctly. Then you may need to continue solving by setting each factor equal to zero.

Watch for the most common mistake: changing the middle sign or forgetting to factor out a common factor first. If the expression is not a perfect cube sum, the formula does not apply. Showing the rewritten cubes before factoring is often the safest way to get full credit.

## Sum of Cubes vs Difference of Cubes

These two formulas look very similar, but the signs are different. For a sum of cubes, the first factor uses addition and the middle term in the second factor is negative: (a + b)(a^2 - ab + b^2). For a difference of cubes, the first factor is subtraction and the middle term is still negative: (a - b)(a^2 + ab + b^2).

## Key Takeaways

- A sum of cubes has the form a^3 + b^3, where both terms are perfect cubes.
- The factoring pattern is a^3 + b^3 = (a + b)(a^2 - ab + b^2).
- Rewrite numbers like 8, 27, or 64 as cubes before you factor, so the pattern is easier to see.
- Check for a greatest common factor first, because that may need to be factored out before the cube pattern appears.
- If your answer does not multiply back to the original expression, the sign or the middle term is probably wrong.

## FAQs

### What is Sum of Cubes in College Algebra?

It is a polynomial expression made by adding two cubic terms, like x^3 + 8 or a^3 + b^3. In College Algebra, the term usually points to the factoring formula a^3 + b^3 = (a + b)(a^2 - ab + b^2).

### How do you factor a sum of cubes?

First, make sure both terms are perfect cubes. Then rewrite the expression in the form a^3 + b^3 and use (a + b)(a^2 - ab + b^2). A quick check by multiplying the factors back together helps you catch sign mistakes.

### What is the difference between sum of cubes and difference of cubes?

The sign in the first factor changes. A sum of cubes uses (a + b)(a^2 - ab + b^2), while a difference of cubes uses (a - b)(a^2 + ab + b^2). The middle term in the second factor changes too, so it is easy to mix them up.

### Do you always factor out the GCF before using sum of cubes?

Yes, if there is a common factor, pull it out first. That makes the remaining polynomial easier to inspect and may reveal the cube pattern more clearly. If you skip this step, you can miss the correct factoring method.

## Related Study Guides

- [5.2 Power Functions and Polynomial Functions](/college-algebra/unit-5/2-power-functions-polynomial-functions/study-guide/iZWPgIKKCptZIm4c)
- [2.6 Other Types of Equations](/college-algebra/unit-2/6-types-equations/study-guide/ksWYqaEjWqE1AhTP)
- [1.5 Factoring Polynomials](/college-algebra/unit-1/5-factoring-polynomials/study-guide/uXutnvo3E4mp3fMx)

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