x^2 - 1 = (x - 1)(x + 1) - Parker Core Knowledge
The Fundamental Factorization: x² – 1 = (x – 1)(x + 1)
The Fundamental Factorization: x² – 1 = (x – 1)(x + 1)
Understanding algebraic expressions is fundamental in mathematics, and one of the most essential and elegant factorizations is that of the difference of squares:
x² – 1 = (x – 1)(x + 1)
Understanding the Context
This equation highlights a powerful identity that not only simplifies quadratic expressions but also opens the door to deeper algebraic concepts such as polynomial factoring, solving equations, and even applications in calculus and number theory.
What Is the Difference of Squares?
The expression x² – 1 is a classic example of a difference of squares, a special form defined by:
a² – b² = (a – b)(a + b)
In this case:
- a = x
- b = 1
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Key Insights
Thus applying the formula, we directly factor:
x² – 1 = (x – 1)(x + 1)
This identity holds true for any real (or complex) value of x, making it a universal shortcut in algebra.
Why Is This Important?
1. Simplifies Quadratic Expressions
Recognizing x² – 1 as a difference of squares allows quick simplification, which is especially useful when expanding or factoring more complex expressions.
2. Solves Equations More Easily
Equations such as x² – 1 = 0 become straightforward when factored:
(x – 1)(x + 1) = 0
Setting each factor to zero gives the solutions x = 1 and x = -1, illustrating how factoring unlocks root finding.
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3. Forms the Basis for Polynomial Identity
This factorization is part of a larger family of identities that are indispensable in algebraic manipulation, calculus (e.g., derivatives and integrals), and even abstract algebra.
Applying the Formula in Real Problems
Example 1: Factoring
Factor the expression x² – 1 step-by-step:
- Identify as difference of squares: a² – b² with a = x, b = 1
- Apply identity: (x – 1)(x + 1)
Thus, x² – 1 = (x – 1)(x + 1)
Example 2: Solving x² – 1 = 0
Using the factorization:
(x – 1)(x + 1) = 0
Solutions:
x – 1 = 0 ⇒ x = 1
x + 1 = 0 ⇒ x = –1
So the roots are x = 1 and x = –1
Example 3: Polynomial Division
This identity helps verify divisibility—for instance, confirming that (x – 1) is a factor of x² – 1 by showing x² – 1 divided by (x – 1) yields (x + 1) exactly.
