Factorise 4x 2 12x 9

Factorising 4x² + 12x + 9: A Comprehensive Guide

This article provides a comprehensive guide on how to factorise the quadratic expression 4x² + 12x + 9. We'll explore various methods, delve into the underlying mathematical principles, and address common questions. Understanding quadratic factorisation is crucial for solving equations, graphing parabolas, and mastering more advanced algebraic concepts. By the end of this article, you'll not only be able to factorise this specific expression but also confidently tackle similar problems.

Introduction to Quadratic Expressions and Factorisation

A quadratic expression is a polynomial of degree two, meaning the highest power of the variable (usually 'x') is 2. It generally takes the form ax² + bx + c, where a, b, and c are constants, and a ≠ 0. Factorising a quadratic expression means rewriting it as a product of two linear expressions. For example, the expression x² + 5x + 6 can be factorised as (x + 2)(x + 3). This process is fundamental in algebra and has numerous applications in various fields.

Our focus here is on factorising 4x² + 12x + 9. This quadratic expression has a leading coefficient (the coefficient of x²) that is not 1, adding a slight layer of complexity compared to simpler cases.

Method 1: Recognising Perfect Square Trinomials

Before jumping into more complex methods, let's check if our quadratic expression is a perfect square trinomial. A perfect square trinomial is a quadratic expression that can be written as the square of a binomial. It follows the pattern (ax + b)² = a²x² + 2abx + b².

Let's examine 4x² + 12x + 9:

• The first term, 4x², is (2x)².
• The last term, 9, is 3².
• The middle term, 12x, is 2 * (2x) * 3 = 12x.

Since the expression fits the pattern of a perfect square trinomial, we can directly factorise it as:

(2x + 3)²

This is the simplest and most efficient method when applicable. Recognizing patterns like perfect square trinomials significantly speeds up the factorisation process.

Method 2: The AC Method (for Quadratics with a Leading Coefficient ≠ 1)

The AC method is a more general approach for factorising quadratic expressions, particularly useful when the leading coefficient (a) is not 1. This method involves finding two numbers that add up to 'b' (the coefficient of x) and multiply to 'ac' (the product of the coefficient of x² and the constant term).

Let's apply the AC method to 4x² + 12x + 9:

1. Identify a, b, and c: In our expression, a = 4, b = 12, and c = 9.

2. Calculate ac: ac = 4 * 9 = 36.

3. Find two numbers that add up to b and multiply to ac: We need two numbers that add up to 12 and multiply to 36. These numbers are 6 and 6.

4. Rewrite the middle term: Rewrite the middle term (12x) as the sum of the two numbers found in step 3: 6x + 6x. Our expression becomes: 4x² + 6x + 6x + 9.

5. Factor by grouping: Group the terms in pairs and factor out the greatest common factor (GCF) from each pair:

   2x(2x + 3) + 3(2x + 3)

6. Factor out the common binomial: Notice that (2x + 3) is a common factor in both terms. Factor it out:

   (2x + 3)(2x + 3) or (2x + 3)²

This method, while more involved than recognizing a perfect square trinomial, provides a systematic approach for factorising any quadratic expression, regardless of its coefficients.

Method 3: Quadratic Formula (for finding roots, then factors)

The quadratic formula is a powerful tool for finding the roots (or zeros) of a quadratic equation. Once you have the roots, you can construct the factors. The quadratic formula is:

x = [-b ± √(b² - 4ac)] / 2a

Applying this to our expression 4x² + 12x + 9 = 0:

1. Identify a, b, and c: a = 4, b = 12, c = 9.

2. Substitute into the quadratic formula:

   x = [-12 ± √(12² - 4 * 4 * 9)] / (2 * 4) x = [-12 ± √(144 - 144)] / 8 x = -12 / 8 = -3/2

Since we only get one root (-3/2), it means the quadratic has a repeated root. This indicates a perfect square trinomial. The factor corresponding to the root -3/2 is (2x + 3). Therefore, the factorised form is (2x + 3)².

Understanding the Connection Between Roots and Factors

The roots of a quadratic equation are the values of x that make the equation equal to zero. There's a direct relationship between the roots and the factors: if 'r' is a root, then (x - r) is a factor. In our case, we have a repeated root of -3/2, leading to the repeated factor (x - (-3/2)) = (x + 3/2). To obtain integer coefficients, we multiply this by 2 resulting in (2x + 3). This reinforces our previous findings.

Expanding the Factored Form to Verify

To verify our factorisation, we can expand the factored form (2x + 3)²:

(2x + 3)(2x + 3) = 4x² + 6x + 6x + 9 = 4x² + 12x + 9

This confirms that our factorisation is correct.

Applications of Quadratic Factorisation

The ability to factorise quadratic expressions is crucial in various mathematical contexts:

• Solving Quadratic Equations: Factorisation allows us to solve quadratic equations easily by setting each factor to zero.
• Graphing Parabolas: The factors of a quadratic expression reveal the x-intercepts (where the parabola crosses the x-axis) of its graph.
• Simplifying Algebraic Expressions: Factorisation can simplify complex algebraic expressions, making them easier to manipulate.
• Calculus: Quadratic factorisation is used in calculus for tasks like finding derivatives and integrals.

Frequently Asked Questions (FAQ)

Q: What if I can't find two numbers that add up to 'b' and multiply to 'ac' in the AC method?

A: If you cannot find such numbers, it means the quadratic expression cannot be factored using integers. In such cases, you might need to use the quadratic formula to find the roots and then construct the factors, or conclude that the expression is prime (cannot be factored).

Q: Is there only one way to factorise a quadratic expression?

A: No, the order of the factors doesn't matter. For example, (2x + 3)(2x + 3) is equivalent to (2x + 3)². However, the resulting factors themselves are unique.

Q: What if the quadratic expression has a leading coefficient of 1 (a=1)?

A: If a=1, the process simplifies. You directly look for two numbers that add up to 'b' and multiply to 'c'.

Q: How can I improve my speed and accuracy in factorising quadratics?

A: Practice is key. The more you practice, the better you'll become at recognizing patterns and applying the various methods efficiently.

Conclusion

Factorising 4x² + 12x + 9 is straightforward once you understand the underlying principles. We've explored three effective methods: recognizing perfect square trinomials, the AC method, and using the quadratic formula. Each method provides valuable insight into the structure and properties of quadratic expressions. Mastering these techniques will significantly enhance your algebraic skills and prepare you for more advanced mathematical concepts. Remember to practice regularly to build confidence and efficiency in your factorisation abilities. By understanding the connections between roots, factors, and the various methods, you can confidently approach any quadratic factorisation problem.