Derivation of the Difference of Cubes Formula

Oct 13, 2025 by wbrum.org 28 views

The difference of cubes formula is a fundamental concept in algebra, used to factorize expressions of the form $a^3 - b^3$ . This formula is essential in solving various mathematical problems, and it's crucial to understand its application and derivation.

What is the Difference of Cubes Formula?

The difference of cubes formula is given by:

a^3 - b^3 = (a - b)(a^2 + ab + b^2)

This formula can be used to factorize expressions of the form $a^3 - b^3$ , where $a$ and $b$ are any real numbers.

Derivation of the Difference of Cubes Formula

To derive the difference of cubes formula, we can start by expanding the expression $(a - b)(a^2 + ab + b^2)$ using the distributive property.

(a - b)(a^2 + ab + b^2) = a(a^2 + ab + b^2) - b(a^2 + ab + b^2)

Expanding each term, we get:

a(a^2 + ab + b^2) = a^3 + a^2b + ab^2

-b(a^2 + ab + b^2) = -a^2b - ab^2 - b^3

Combining like terms, we get:

a^3 + a^2b + ab^2 - a^2b - ab^2 - b^3 = a^3 - b^3

Therefore, we have derived the difference of cubes formula:

a^3 - b^3 = (a - b)(a^2 + ab + b^2)

Applying the Difference of Cubes Formula

Now that we have derived the difference of cubes formula, let's apply it to the given options.

Option A: $x^6 - 27$

We can rewrite $x^6 - 27$ as $(x^2)^3 - 3^3$ . Using the difference of cubes formula, we get:

(x^2)^3 - 3^3 = (x^2 - 3)((x^2)^2 + x^2 \cdot 3 + 3^2)

Simplifying, we get:

(x^2 - 3)(x^4 + 3x^2 + 9)

Therefore, option A is a difference of cubes.

Option B: $x^{15} - 36$

We can rewrite $x^{15} - 36$ as $(x^5)^3 - 6^3$ . Using the difference of cubes formula, we get:

(x^5)^3 - 6^3 = (x^5 - 6)((x^5)^2 + x^5 \cdot 6 + 6^2)

Simplifying, we get:

(x^5 - 6)(x^{10} + 6x^5 + 36)

Therefore, option B is not a difference of cubes.

Option C: $x^{16} - 64$

We can rewrite $x^{16} - 64$ as $(x^4)^4 - 4^4$ . Using the difference of cubes formula, we get:

(x^4)^4 - 4^4 = (x^4 - 4)((x^4)^2 + x^4 \cdot 4 + 4^2)

Simplifying, we get:

(x^4 - 4)(x^8 + 4x^4 + 16)

Therefore, option C is not a difference of cubes.

Option D: $x^5 - 125$

We can rewrite $x^5 - 125$ as $(x)^5 - 5^3$ . Using the difference of cubes formula, we get:

(x)^5 - 5^3 = (x - 5)((x)^2 + x \cdot 5 + 5^2)

Simplifying, we get:

(x - 5)(x^2 + 5x + 25)

Therefore, option D is a difference of cubes.

Conclusion

In conclusion, the difference of cubes formula is a fundamental concept in algebra, used to factorize expressions of the form $a^3 - b^3$ . We have derived the formula and applied it to the given options. We have found that options A and D are differences of cubes, while options B and C are not.

Key Takeaways

The difference of cubes formula is given by $a^3 - b^3 = (a - b)(a^2 + ab + b^2)$ .
The formula can be used to factorize expressions of the form $a^3 - b^3$ .
The formula can be derived by expanding the expression $(a - b)(a^2 + ab + b^2)$ using the distributive property.
The formula can be applied to various mathematical problems, including factoring expressions and solving equations.

Final Answer

In our previous article, we discussed the difference of cubes formula and its application to various mathematical problems. In this article, we will answer some frequently asked questions about the difference of cubes formula.

Q: What is the difference of cubes formula?

A: The difference of cubes formula is a fundamental concept in algebra, used to factorize expressions of the form $a^3 - b^3$ . The formula is given by:

a^3 - b^3 = (a - b)(a^2 + ab + b^2)

Q: How do I apply the difference of cubes formula?

A: To apply the difference of cubes formula, you need to identify the expression $a^3 - b^3$ and then factor it using the formula. For example, if you have the expression $x^6 - 27$ , you can rewrite it as $(x^2)^3 - 3^3$ and then apply the formula:

(x^2)^3 - 3^3 = (x^2 - 3)((x^2)^2 + x^2 \cdot 3 + 3^2)

Simplifying, you get:

(x^2 - 3)(x^4 + 3x^2 + 9)

Q: What are some common mistakes to avoid when applying the difference of cubes formula?

A: Some common mistakes to avoid when applying the difference of cubes formula include:

Not identifying the expression $a^3 - b^3$ correctly
Not factoring the expression correctly
Not simplifying the expression correctly

Q: Can I use the difference of cubes formula to factorize expressions of the form $a^3 + b^3$ ?

A: No, the difference of cubes formula is only used to factorize expressions of the form $a^3 - b^3$ . If you have an expression of the form $a^3 + b^3$ , you will need to use a different formula, such as the sum of cubes formula.

Q: How do I derive the difference of cubes formula?

A: To derive the difference of cubes formula, you can start by expanding the expression $(a - b)(a^2 + ab + b^2)$ using the distributive property. This will give you:

(a - b)(a^2 + ab + b^2) = a(a^2 + ab + b^2) - b(a^2 + ab + b^2)

Expanding each term, you get:

a(a^2 + ab + b^2) = a^3 + a^2b + ab^2

-b(a^2 + ab + b^2) = -a^2b - ab^2 - b^3

Combining like terms, you get:

a^3 + a^2b + ab^2 - a^2b - ab^2 - b^3 = a^3 - b^3

Therefore, you have derived the difference of cubes formula:

a^3 - b^3 = (a - b)(a^2 + ab + b^2)

Q: What are some real-world applications of the difference of cubes formula?

A: The difference of cubes formula has many real-world applications, including:

Factoring expressions in algebra
Solving equations in algebra
Finding the roots of a polynomial equation
Factoring expressions in calculus

Q: Can I use the difference of cubes formula to factorize expressions of the form $a^n - b^n$ ?

A: No, the difference of cubes formula is only used to factorize expressions of the form $a^3 - b^3$ . If you have an expression of the form $a^n - b^n$ , you will need to use a different formula, such as the difference of powers formula.

Conclusion

In conclusion, the difference of cubes formula is a fundamental concept in algebra, used to factorize expressions of the form $a^3 - b^3$ . We have answered some frequently asked questions about the difference of cubes formula and provided examples of its application. We hope this article has been helpful in understanding the difference of cubes formula and its real-world applications.