Mathematics
Grade 12
15 min
Factor sums and differences of cubes
Factor sums and differences of cubes
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Introduction & Learning Objectives
Learning Objectives
Identify binomials that are sums or differences of perfect cubes.
Recall and correctly state the formulas for factoring a³ + b³ and a³ - b³.
Apply the appropriate formula to factor cubic binomials.
Factor out a Greatest Common Factor (GCF) before applying the sum or difference of cubes formulas.
Factor expressions with higher-order exponents that fit the cubic pattern, such as x⁶ - y⁹.
Verify their factorization by expanding the resulting factors.
Use factoring to find the real and complex roots of cubic equations.
How can you algebraically deconstruct a perfect 3D cube into its fundamental factors? 🧊 Let's find out!
This tutorial focuses on two special factoring patterns: the sum of cubes and the difference of cubes. Mastering these formulas is...
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Key Concepts & Vocabulary
TermDefinitionExample
Perfect CubeA number or expression that is the result of multiplying a number or expression by itself three times.27 is a perfect cube because 3³ = 27. The term 64x⁶ is a perfect cube because (4x²)³ = 64x⁶.
Cubic BinomialA polynomial with two terms where the highest degree of the variable is three.x³ - 8
Sum of CubesA cubic binomial that can be written in the form a³ + b³.y³ + 125, which can be written as y³ + 5³.
Difference of CubesA cubic binomial that can be written in the form a³ - b³.8x³ - 1, which can be written as (2x)³ - 1³.
Irreducible Quadratic FactorA quadratic factor that cannot be factored further using real numbers. The quadratic part of the sum/difference of cubes formula is always irreducible over the reals.In the factorization of x³ + 8 = (x+2)(x²-2x...
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Core Formulas
Sum of Cubes Formula
a³ + b³ = (a + b)(a² - ab + b²)
Use this formula when you have two perfect cubes added together. The resulting factors are a binomial and a trinomial.
Difference of Cubes Formula
a³ - b³ = (a - b)(a² + ab + b²)
Use this formula when you have one perfect cube subtracted from another. The resulting factors are a binomial and a trinomial.
SOAP Mnemonic for Signs
SOAP: Same, Opposite, Always Positive
A memory aid for the signs in the formulas. The first sign in the factored form is the **S**ame as the original expression. The second sign is the **O**pposite. The third sign is **A**lways **P**ositive.
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Sign Up Free to ContinueSample Practice Questions
Challenging
Find all roots, real and complex, for the equation x³ + 27 = 0.
A.x = -3, x = 3/2 ± (3i√3)/2
B.x = 3, x = -3/2 ± (3i√3)/2
C.x = -3, x = 3 ± 3i√3
D.x = -3 only
Challenging
Factor the expression x⁶ - 64 completely into linear and irreducible quadratic factors over the real numbers.
A.(x³ - 8)(x³ + 8)
B.(x - 2)(x + 2)(x² + 2x + 4)(x² - 2x + 4)
C.(x² - 4)(x⁴ + 4x² + 16)
D.(x - 2)³(x + 2)³
Challenging
Given that (3x - 2) is a factor of 27x³ - 8, find the complex roots of the equation 27x³ - 8 = 0.
A.x = -1/3 ± (i√3)/3
B.x = 1/3 ± (i√3)/3
C.x = -2/3 ± (2i√3)/3
D.x = 2/3 ± (i√3)/3
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